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
3 - launch IDVD and try again.
NOTE: In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.