To specify the portion of video you want to show:
Make sure the menu that uses the background video you want to adjust is showing in the iDVD window.
With your pointer over the menu (and with no buttons or text objects selected), press Command-I to open the Menu Info window.
Click the small triangle above the Loop Duration slider. (You see the triangle only when custom video is in the background well.)
The slider control becomes the Background Movie Start/End control (shown below), and the slider is split in half.
Move the left half of the slider, which plays the movie in the iDVD window, until you see the frame where you want the background video to start.
Move the right half of the slider until you see the frame where you want the background video to end.
Click the triangle again to return to the Loop Duration slider.