Here are ways to prepare your iMovie for iDVD:
To export your iMovie directly to an iDVD project, within iMovie HD choose Share > iDVD. The iDVD application opens, with your project in the iDVD window. If you’ve added chapter markers to your iMovie, iDVD uses them to create a scene selection menu automatically.
When iDVD opens, it selects a default theme, which appears in the iDVD window. If the iMovie you shared has chapter markers, you see two buttons on the iDVD menu, one that says “Play Movie” and another that says “Scene Selection.” If the movie does not have chapter markers, you see one button with the name of the movie you added. From here, you can begin editing your iDVD project, including selecting a different theme if you want.
To save your iMovie as a QuickTime movie that you can import into an iDVD project later, choose File > Export in iMovie HD. Click QuickTime, choose Full Quality from the pop-up menu, and then click Share.
Your movie is saved as a QuickTime movie in your Movies folder, making it available in the Media pane of iDVD. The movie is self-contained (it includes all the source files). Chapter markers are not preserved, so iDVD cannot create a scene selection menu for a QuickTime movie.