There was a bug in Flash Player 9, where the text of a Flex Menu control would not show up when your application was in Full Screen mode. Actually, you would see the main menu, but, submenus would have no text visible. This problem was fixed in Flash Player 10. If you grab the Flash Player 10 alpha, you should see that the problem has been fixed.
However, naturally, you can’t depend on all of your users having Flash Player 10 immediately. You can make a minor change to a Flex Menu to work around the text problem. Just set the openDuration style for the Menu to 0. This will remove the Tween from the opening of your Menus, which may not be desireable, but, at least your text will show up. Here’s a small sample snippet:
var categoryMenu:Menu = Menu.createMenu(this, xmlFile, false);
For reference and more information, you can see the Flex Bug that was filed related to this problem. SDK-15007