I don`t have a Mac myself so I did some research and found a couple of options for you try;
click on the blue apple at the left top corner > about this mac > more info
click on firewire and see it the system profiler recognize the device you plugged into.
if that camera is detected, then just fire up your iMovie and switch the button under the black screen area to camera icon instead of scissor icon.
make sure your camera is on vtr / playback mode and not in camera mode.
if camera or other device is not detected, check using other cable first, and if it still not mounted, visit apple and let them check it for you, or
For most situations, a restart is sufficient. If the computer has stopped responding, try each one of these steps, one by one. Test in between steps to see if it has worked. If one step works, don't worry about the next, as you're up and running! Only go on to reset the SMC if you've tried all of the steps listed here and the computer still isn't working.
1. Force Quit (Option-Command-Escape)
2. Restart (Control-Command-Power)
3. Force Shut Down (press the power button for 10 seconds)
Only perform an SMC reset on the computer if none of the above steps resolved the situation. If this is the case, see below for instructions on locating the reset button or key combination for your model of computer. If your computer is earlier than the models listed here, see "Resetting PowerBook and iBook Power Management Unit (PMU)" or "PowerBook: Resetting Power Management Unit (PMU)".
MacBook, MacBook Pro
1. If the computer is on, turn it off.
2. Disconnect the AC Adapter and remove the computer's battery.
3. Press and hold down the power button for 5 seconds and then release the button.
4. Reconnect the battery and AC Adapter.
5. Press the Power button to restart the computer.
I hope one of the above helps you get sorted and let us know how it works out for you.