I had a dual G5, and the process i used went something like:
1. Open file in Quicktime player, File -> Export or save to DVD file, or mpeg (or something)
2. Open Idvd, drag the files created in 1. to idvd.
3. Click burn.
(I can't remember the exact file types/ but use a short clip and you can test the whole thing in a few minutes.)