To capture footage using your iSight camera (the built-in camera on the iMac) inside iMovie, press the Import button at the top of iMovie, which is a downward-facing arrow.
When you do, a window will pop up. Choose FaceTime Camera (Built-in) under the Cameras section on the left side of the window.
You can choose which Event you want your recorded footage to save under by selecting the Import to: drop-down menu at the top of the import window. You can also make a new event from this menu.
When you are ready to record, press the red Record button near the bottom of the window.
You will know you are recording when you see the counter going up in the right hand corner. The Record button will also be glowing red when you are recording.
Press the Record button again to stop recording.
If you want to capture more footage, press the Record button again. If you are done, press the Close button in the bottom right corner of the window, and you will be taken back to your project.
Your recorded iSight footage will now be in your specified Event.