Mac Forums

Okay so I just got a new 2011 iMac and I got to playing the Sims 3 and it just didn't look right.
Here are my computers specs:

Processor 2.5 GHz Intel Core i5
Memory 4 GB 1333 MHz DDR3
Graphics AMD Radeon HD 6750M 512 MB
Software Mac OS X Lion 10.7.2 (11C74)

But I'm confused about one thing. When I went to run the game, and I got to the neighborhood a message came up that said. "No supported Video Graphics Card Detected" or something like that. What I need to know is why would it say that? And how do I improve the graphics? Like what settings do you suggest on Sims 3 for better performance?

And would I be able to add other Sims expansions to it?

Any help would be great!