Try "export" under the file menu, the "Quicktime movie" option under that. Save the movie as NOT selfcontained. Remember the name that you saved the movie under.
Then, open iDVD, and go to the media tab under customize. Find the movie file that you saved above, and drag it to the screen in iDVD. If you had chapter markers, and saved the movie with chapter markers, the 'scene selection' will also be available on the iDVD main screen.
Hope that helps...