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You need to use the FireWire (or i-link dv socket, as it's only a 4 pin firewire port) found on the back under the cover near your USB socket and power socket, as shown here:
JVC-GR-D270_back-ports.jpg

It's best to use FireWire with things like this if you can, lots of threads explaining why. Plus iChat will only use FireWire cameras and not USB cameras unless you buy a plugin for it.

I have a similar older JVC camera I use at work, actually just used it to record a school production at work (iMovie'd and iDVD'd ofcourse!) I hope your's has better image quality and light control than the one I use!

Hope this helps!
 
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