Te aconsejo que te leas bien los errores antes de continuar. Si pone undefined method es que no existe el método, ya que en este caso debería ser:
$sql_statement = "SELECT user.Host, user.User, user.Password, Select_priv as `Select priv`, Insert_priv as `Insert Priv`, Update_priv as `Update priv`, Delete_priv as `Delete priv`, Create_priv as `Create priv`, Reload_priv as `Reload priv`, Shutdown_priv as `Shutdown priv`, Process_priv as `Process priv`, File_priv as `File priv`, Grant_priv as `Grant priv`, References_priv as `References priv`, Index_priv as `Index priv` FROM user LEFT JOIN tables_priv ON user.User=tables_priv.User ORDER BY user.User" ;
// Generate report
$pdf->mysql_report($sql_statement, false, $attr );
/*!!! Very Important: after having done all the work of
setting up the SQL don't forget to output the PDF else
you just get a blank page !!!*/
$pdf->Output();
El código está extraido de la web oficial de fPDF.
http://www.fpdf.org/en/script/script21.phpSalu2