I presume this is a miniDV tape based camcorder...

Good you have the 4-pin to 6-pin firewire cable... Connect the 4-pin side to the camcorder DV port and the 6-pin side to the Mac's firewire 400 port. Be sure the camcorder is also plugged into the AC wall power. The camcorder needs to be in "Play" or "VCR" mode. Since we don't know which camcorder you have, you make us cover all bases...

Launch iMovieHD. When the blue "Camera Connected" screeen appears, click on the "Import" or the rewind button, them click Import....

DO NOT PLUG IN FIREWIRE and USB AT THE SAME TIME. USB is for transferring stills only.