The problem is that JVC screwed up their implementation of Firewire. You *have* a digital camcorder *but* it does not follow the Firewire I/O specs.
Remember, saying "my camcorder has a Firewire port" is like saying "my VTR has an RS-422 port". Firewire merely specifies the interface specs. It's up to the manufacturer -- whether hardware or software -- to implement them.
Sell your JVC. Find a camcorder like the Panasonic PV-DV900 (discontinued, but about $750 on closeout at Sears) or if you're into Canon, one of their mini-DV cameras mentioned as being compatible with iMovie.
All the DVMC-DA1 does is change analog video into Firewire DV I/O that iMovie can recognize.