I'm having a similar problem...
Here's my setup ---
Composite video cable out (yellow, white, red) from my HDTV to the ADVC-55... then a FireWire 400 to 800 cable (quality cable, from Monoprice.com) into my MacBook Pro (unibody, which only has FW800).
The converter shows the Red "no signal" light and of course, I'm not able to record any video through iMovie or QuickTime Player.
My theories...
1. Do I need to get an AC Adapter for the converter? It seems like the FW800 port would power it, but I could be wrong.
2. Is the High-Def origin signal a problem? I doubt it, as I've tried both my cable box and output from my TV and I assume it down-converts.
Any help would be appreciated. Thanks!