"There were warnings during the project validation. It is recommended that you fix these problems before burning the project."
My guess is you left something out of one or more drop zones in the menu layout. This is OK, as long as you don't mind a blank spot in those areas of your menu. When you get that error message, just click "Continue burning". Notice the word "recommended" in the warning message. Recommended is not to be confused with necessary. In this case, filling the drop zones is not necessary.
The way to check what's missing is to click on the "Show the DVD Map" button at the bottom of your menu window. It's the 1st button on the left, that has 3 tiny boxes on it. When the map window opens, look for any menu thumbnails that have a little triangle with an exclamation point inside. Pass your mouse arrow over it, and you will get the error message. My guess is the error you'll see is "Menu has empty drop zones." (It might be something else, too, but empty drop zones are usually the culprit.)
Click the return button at the bottom right of the Map win dow to return to your menu layout. Here, you can either fill the drop zones, or leave them empty and do your burn.
The iDVD themes provide drop zones for what the theme designers figured would make nice menus. The warning you get before you burn is simply to let you know that you haven't "fulfilled the recommended drop zone requirements" as deemed by the Apple designers. But be aware that in some of the themes, the drop zones don't look good without something in them.
You could eliminate all the drop zones from your menu, too, if you wanted.
In your menu window, click the little "i" button at the bottom left of the window. In the menu info box that comes up, there is a checkbox at the bottom for "Show drop zones and related graphics". You can uncheck this box.
