iDVD is picky in regard to what Video-Codec it can use (or not).
.avi, .wmv, .mp4 etc are NO CODEC - they are Containers and can hold Video-Codecs that works with iDVD or not.
Open Your .avi file (container) with QuickTime-player
Then open Inspector [cmd+I]
Then read -
• Video Format (Codec) : nnnnnnnnnnnnnnn
• Frames per second : yyyyyyyyyyy
WHAT does they read ?
Yours Bengt W