JMenu Bar on the Mac

I'm trying to set JMenuBar on a Mac, but each time I try to set it, it comes up in a regular frame. Here;s my code:

if (vers.indexOf("mac") != -1)
{
//System.setProperty("apple.laf.useScreenMenuBar", "true");
System.setProperty ("com.apple.macos.useScreenMenuBar","true");
macFrame=new JFrame();
menu=new JMenuBar();
createMenus();
macFrame.setJMenuBar(menu);
macFrame.setVisible(true);
macFrame.pack();


}

Its very similar to the examples of I've but it still doesn't work, am I missing something? And the programming language is Java.