Try this basic troubleshooting fix:
1 - delete the IDVD preference file, com.apple.iDVD.plist, that resides in your
User/Home/Library/ Preferences folder.
2 - delete IDVD'S cache file, Cache.db, that is located in your
User/Home/Library/Caches/com.apple.iDVD folder.
3 - launch IDVD and try again.
NOTE: In Lion, Mountain Lion and Mavericks the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and press the Return key - 10.7: Un-hide the User Library folder.
If you're running Mavericks, 10.9, go to your Home folder and use the View ➙ Show View Options menu to bring the this window:
It the above doesn't help Boot into Safe Mode, Mac OS X: Starting up in Safe Mode, and try iDVD there. Then reboot normally and try again.