Here are ways to add content to the Media pane:
Drag folders (not individual files) of music, images, and movies from your hard disk or an external disk directly to the Media pane.
Make sure to click the appropriate button at the top of the Media pane (Audio, Photos, or Movies) before adding the folder.
Specify other folders for iDVD to monitor. iDVD automatically adds the movies they contain to the Media pane. Any time you add a movie to the monitored folder, it automatically appears in the Media pane. To add a folder:
Choose iDVD > Preferences.
Click Movies.
Click the Add button next to the list titled “Look for my movies in these folders.”
Navigate to the folder you want to add, select it, and click Open. To select more than one folder, hold down the Shift key while you click.
The folder name appears in the list on the left in the Preferences window and in the Movies pane. You can add as many folders as you want.