First make sure your camera is set to the VTR (palyback) mode...sounds like it already is if you can see and control video on it.
Make sure your cable is plugged into the correct port on the camera (it should be marked "DV" and be in the center of the 3 ports under the cover next to the lens) and computer and that your are not using a USB cable or plug.
Once the cables are conncected turn on the camcorder and start up iMovie (in that order).
If iMovie doesn't see the camera right away try unplugging the camera...trun it off and back on...leave it plugged into the computer and restart the computer.
On the bottom left hand side of the screen there is an icon of a camera and scissors with a little slide button between them.
click the camera. The screen should change and an import button appear above the palyer controls. (It will be grayed out if it doesn't see the camera.)
If it sees the camera click the import button to start importing your footage right where the tape is qued up or use the Play/FF/RW/Pause buttons to que up to where you want to import from.
At least that's how it is on Ver5 of iMovie HD. What version are you using?
There are step by step instruction at
www.apple.com/support/imovie
Good luck. Let me know how it goes.