It's a problem with your computer, more likely than not. Did you purchase it within the last 14 days? If so, you can return it and get a new one, same configuration, few questions asked, or get a full refund. Before you do that, however, check out one thing: go under the Apple menu to "About This Mac" and then click "More Info..." then "System Report...". Look in the Hardware section under USB and see if you see "FaceTime HD Camera (Built-in)" installed. If not, then the system is simply not recognizing the camera.
Take it back to the Apple Store as soon as possible. Even if you're not in your 14 day 'grace period' you still have (at least) a one year warranty and they should be able to repair or replace the model.
Good luck,
Clinton