Problema Al Ejecutar Un .jar

Estimados

temgo un problema estoy desarrollando un examen en netbenas como applicacion de escritorio, tengo las clases main creada y el frame donde esta el diseño que les describo linea abajo, pero al momento de ejecutar el jar. me sale un sms indica could not find the main class:classes.principal ya le asiganado la clase principal y nada.. cuando ejecuta desde la misma plataforma netbeans el proyecto corre sin problemas...
que creen que deberia de hacer...

*************************************************

package Ventanas;

import java.sql.*;
import Clases.*;
import java.awt.Toolkit;
import javax.swing.JOptionPane;

//Daniel

public class Examen extends javax.swing.JFrame {

Connection con =null;
PreparedStatement s;
String url= "jdbc:mysql://localhost:3306/examenfonobus2014";
String user="root";
String clave="mathias1";

private static final long serialVersionUID = 1L;

public Examen() {
initComponents();
question.setText(p.getPregunta(posicion));
String[] a = r.setRespuesta(posicion);
buttonGroup1.clearSelection();
opc1.setText(a[0]);
opc2.setText(a[1]);
opc3.setText(a[2]);
opc4.setText(a[3]);
opc1.requestFocus();
regresar.setEnabled(false);
TerminarExamen.setEnabled(false);
}
int posicion = 0;
Respuestas r = new Respuestas();
Preguntas p = new Preguntas();
Object[] select = {"","","","",""};

@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {

buttonGroup1 = new javax.swing.ButtonGroup();
jPanel1 = new javax.swing.JPanel();
jLabel1 = new javax.swing.JLabel();
jLabel2 = new javax.swing.JLabel();
question = new javax.swing.JLabel();
opc1 = new javax.swing.JRadioButton();
opc2 = new javax.swing.JRadioButton();
opc3 = new javax.swing.JRadioButton();
opc4 = new javax.swing.JRadioButton();
TerminarExamen = new javax.swing.JButton();
regresar = new javax.swing.JButton();
avanzar = new javax.swing.JButton();
jButton1 = new javax.swing.JButton();
jSeparator1 = new javax.swing.JSeparator();

setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
setTitle("EXAMEN FONOBUS");
setCursor(new java.awt.Cursor(java.awt.Cursor.DEFAULT_CURSOR));

jPanel1.setBackground(new java.awt.Color(0, 0, 51));
jPanel1.setBorder(javax.swing.BorderFactory.createTitledBorder("Examen"));
jPanel1.setForeground(new java.awt.Color(255, 255, 51));
jPanel1.setAutoscrolls(true);
jPanel1.setMaximumSize(new java.awt.Dimension(2147483647, 2147483647));
jPanel1.setMinimumSize(new java.awt.Dimension(0, 0));
jPanel1.setName(""); // NOI18N

jLabel1.setFont(new java.awt.Font("Tahoma", 0, 24)); // NOI18N
jLabel1.setForeground(new java.awt.Color(255, 255, 0));
jLabel1.setHorizontalAlignment(javax.swing.SwingConstants.CENTER);
jLabel1.setText("Examen Fonobus Marzo 2014");
jLabel1.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);

jLabel2.setFont(new java.awt.Font("Tahoma", 0, 18)); // NOI18N
jLabel2.setForeground(new java.awt.Color(255, 255, 0));
jLabel2.setText("Preguntas:");
jLabel2.setFocusCycleRoot(true);
jLabel2.setHorizontalTextPosition(javax.swing.SwingConstants.LEADING);

question.setForeground(new java.awt.Color(255, 255, 0));
question.setText("1.- Pregunta 1?");

opc1.setBackground(new java.awt.Color(255, 255, 204));
buttonGroup1.add(opc1);
opc1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
opc1ActionPerformed(evt);
}
});

opc2.setBackground(new java.awt.Color(255, 255, 204));
buttonGroup1.add(opc2);
opc2.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
opc2ActionPerformed(evt);
}
});

opc3.setBackground(new java.awt.Color(255, 255, 204));
buttonGroup1.add(opc3);
opc3.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
opc3ActionPerformed(evt);
}
});

opc4.setBackground(new java.awt.Color(255, 255, 204));
buttonGroup1.add(opc4);
opc4.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
opc4ActionPerformed(evt);
}
});

TerminarExamen.setText("Terminar Examen");
TerminarExamen.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
TerminarExamenActionPerformed(evt);
}
});

regresar.setText("<< Regresar");
regresar.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
regresarActionPerformed(evt);
}
});

avanzar.setText("Avanzar >>");
avanzar.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
avanzarActionPerformed(evt);
}
});

jButton1.setText("Salir");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton1ActionPerformed(evt);
}
});

javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
jPanel1.setLayout(jPanel1Layout);
jPanel1Layout.setHorizontalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addGap(0, 0, Short.MAX_VALUE)
.addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addComponent(regresar)
.addGap(18, 18, 18)
.addComponent(avanzar)
.addGap(15, 15, 15))
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addComponent(TerminarExamen, javax.swing.GroupLayout.PREFERRED_SIZE, 126, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(44, 44, 44))))
.addGroup(jPanel1Layout.createSequentialGroup()
.addGap(32, 32, 32)
.addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(opc1)
.addComponent(question)
.addComponent(opc2)
.addComponent(opc3)
.addComponent(opc4)
.addComponent(jButton1))
.addContainerGap(859, Short.MAX_VALUE))
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addGap(136, 136, 136))
.addComponent(jLabel2, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addComponent(jSeparator1)
);
jPanel1Layout.setVerticalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(jPanel1Layout.createSequentialGroup()
.addContainerGap()
.addComponent(jLabel1)
.addGap(18, 18, 18)
.addComponent(jSeparator1, javax.swing.GroupLayout.PREFERRED_SIZE, 10, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(14, 14, 14)
.addComponent(jLabel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addGap(27, 27, 27)
.addComponent(question)
.addGap(26, 26, 26)
.addComponent(opc1)
.addGap(18, 18, 18)
.addComponent(opc2)
.addGap(18, 18, 18)
.addComponent(opc3)
.addGap(18, 18, 18)
.addComponent(opc4)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 42, Short.MAX_VALUE)
.addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(regresar)
.addComponent(avanzar))
.addGap(18, 18, 18)
.addComponent(TerminarExamen)
.addGap(23, 23, 23))
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addComponent(jButton1)
.addContainerGap())))
);

javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addContainerGap())
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addContainerGap())
);

pack();
}// </editor-fold>

private void avanzarActionPerformed(java.awt.event.ActionEvent evt) {
if(posicion ==3){
avanzar.setEnabled(false);
TerminarExamen.setEnabled(true);
}
if (posicion <5){
regresar.setEnabled(true);
posicion ++;
question.setText(p.getPregunta(posicion));
String[] a = r.setRespuesta(posicion);
buttonGroup1.clearSelection();
opc1.setText(a[0]);
opc2.setText(a[1]);
opc3.setText(a[2]);
opc4.setText(a[3]);
opc1.requestFocus();
}else{
Toolkit.getDefaultToolkit().beep();
}
}

private void regresarActionPerformed(java.awt.event.ActionEvent evt) {
if(posicion > 0){
posicion --;
}else{
}
if(posicion == 0){
regresar.setEnabled(false);
}
if(posicion > -1){
avanzar.setEnabled(true);
question.setText(p.getPregunta(posicion));
String[] a = r.setRespuesta(posicion);
buttonGroup1.clearSelection();
opc1.setText(a[0]);
opc2.setText(a[1]);
opc3.setText(a[2]);
opc4.setText(a[3]);
opc1.requestFocus();
}else{
Toolkit.getDefaultToolkit().beep();
}
}

private void TerminarExamenActionPerformed(java.awt.event.ActionEvent evt) {
int calificacion = 0;
for(int i = 0; i < 5; i++){
if(select.equals(r.getRespuesta(i))){
calificacion = calificacion + 1;
}
}
calificacion = calificacion * 2;
JOptionPane.showMessageDialog(null, " Tu Calificacion es " + calificacion);
regresar.setEnabled(false);
TerminarExamen.setEnabled(false);

System.exit(0);
}

private void opc1ActionPerformed(java.awt.event.ActionEvent evt) {
select[posicion]= opc1.getText();
}

private void opc2ActionPerformed(java.awt.event.ActionEvent evt) {
select[posicion]= opc2.getText();
}

private void opc3ActionPerformed(java.awt.event.ActionEvent evt) {
select[posicion]= opc3.getText();
}

private void opc4ActionPerformed(java.awt.event.ActionEvent evt) {
select[posicion]= opc4.getText();
}

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
System.exit(0);
}

/**
* @param args the command line arguments
*/
public static void main(String args[]) {
/* Set the Nimbus look and feel */
//<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
/* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
try {
for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
javax.swing.UIManager.setLookAndFeel(info.getClassName());
break;
}
}
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(Examen.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>

/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
new Examen().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton TerminarExamen;
private javax.swing.JButton avanzar;
private javax.swing.ButtonGroup buttonGroup1;
private javax.swing.JButton jButton1;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JPanel jPanel1;
private javax.swing.JSeparator jSeparator1;
private javax.swing.JRadioButton opc1;
private javax.swing.JRadioButton opc2;
private javax.swing.JRadioButton opc3;
private javax.swing.JRadioButton opc4;
private javax.swing.JLabel question;
private javax.swing.JButton regresar;
// End of variables declaration
}