Connect the camcorder and then start up iMovie.
Ensure you are in the correct camcorder mode usb-PC.
Ensure the camcorder is plugged into AC Power.
See if the computer detects the device. Either mounted as an external volume or in System Profiler.
Take a look at http://support.apple.com/kb/HT1793 and http://support.apple.com/kb/TS3720
Depending on how many clips you have on the camcorder, it may take awhile to detect. I have seen instances where 300 clips took 15mins for iMovie to parse and then display the camcorder.
On the rare circumstance, you may have to initialize the camcorder for iMovie to work with it. I have yet to reproduce this issue, but I have seen others that experienced this.