How can I read the video input from a game cube on the iMac screen
If you have an iMac with a Mini-DisplayPort or Thunderbolt port (21/27 inches) . 

You can use your iMac in target screen mode using this kind of adapter : Mini-DisplayPort to Composite 

If your iMac is older (20/24 inches) you won't be able to use your iMac with your GameCube.