Could be a bad cable. You can rule this out if your camera will work on a different Mac with this cable.
1) Camera must be plugged in to AC power (not batteries only).
2) Camera usually should be set to PLAY mode (but check your manual)
iMovie should sense your camera and display an import screen. If it does not, you can use FILE/IMPORT FROM CAMERA.
Inexpensive Firewire cables are available here.