Realtime shadow not working on Mac with Radeon HD

Hi,

I have compiled my game using unity 4 pro. The realtime shadows looks great on my windows machine and my old macbook pro running a nvidia geforce card. However when i run the app on my newer mac machine the realtime shadow disappeared. It has a Radeon HD 5870 card. Does anyone have a similar problem? According to [this][1] page the card should work. The mac with the radeon card have unity basic but not pro installed, but i cannot imagine that being the problem. Any help would be greatly appreciated.

TX [1]: http://docs.unity3d.com/Documentation/Manual/Shadows.html