Unfortunately, I don't have a Mac. So, I can't debug this issue much. From what I can see, the way I'm rendering HTML panels in 3D2D doesn't work on Mac. Have you tried going into the theater and enabling fullscreen mode? My guess is that's the only way it will work on a Mac.

Is there anyway I can help to debug the problem?

My guess is that this issue may be resolved when Awesomium 1.7 is released for Mac. Awesomium is what Garry's Mod uses for rendering HTML panels in-game. If you want to help, I need someone to tell me whether or not fullscreen mode works on Mac.

no it don't

Try following these solutions http://www.pixeltailgames.com/cinema/help.php

Full screen does not work and your tips don't work Sam

If they did work I wouldn't be here

If you had said that you tried them, I wouldn't have asked. It seems most people are able to fix this problem by disabling hardware acceleration on flash videos. If that doesn't work, I'm not sure what will. The best fix would likely be installing Boot Camp and running Windows, sadly.