Help with Mac Cam in Yahoo Messenger

So I just got a MacBook Pro a couple months ago, and I am having issues using webcam chat in Yahoo Messenger for Mac. The built in iSight cam doesn't seem to turn on (no green light) when I try to use it, and the person on the other end just sees a black screen for my webcam.

I have googled and googled this, but have not found a solution yet. Any help?

Thanks!