Try converting the footage to MPEG first. DVDSP is less forgiving than iDVD, which is simply converting it for you. You may prefer to ahve more control over the process though, in which case run it through compressor and get it into the right format first.
Are you hoping to make a HD disc? Or are you wanting to downconvert to SD? Remember, DVDSP can create HD DVDs, but you may not be able to burn them, or play them.