//我使用jcreator2.00_007+j2sdk1.3.1_02
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MDITest extends JFrame 
{
private JButton btnAdd=new JButton("添加内部窗体");
private JDesktopPane desktopPane=new JDesktopPane();
private int windowCount=1;

public MDITest()
{
setTitle("桌面窗格和内部窗体测试");
setSize(500,500);
addWindowListener(new WindowAdapter()
{
public void windowClosing(WindowEvent evt)
{
dispose();
System.exit(0);
}
});

Container contentPane=getContentPane();
contentPane.add(btnAdd,BorderLayout.NORTH);
contentPane.add(desktopPane,BorderLayout.CENTER);

desktopPane.setLayout(new FlowLayout());

btnAdd.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
JInternalFrame jif=new JInternalFrame(
"内部窗体"+windowCount,
true,
true,
true,
true);
jif.setPreferredSize(new Dimension(250,100));
desktopPane.add(jif);
desktopPane.revalidate();

}
});
}



public static void main(String[] args)
{
(new MDITest()).show();
}


}

你那个internalframe必须要show，如下：
desktopPane.add(jif);
jif.show();