mercury engine software only. How can i change it to hardware?

radeon cards aren't supported, only a couple radeon GPUs for OSX in last year's macbook pro's are supported.
A little homework before purchasing and you would have found you needed an nvidia cuda-capable card.

The following cards are officially supprted:

GeForce GTX 285 (Windows and Mac OS)
GeForce GTX 470 (Windows)
GeForce GTX 570 (Windows)
GeForce GTX 580 (Windows)
NVIDIA® Tesla C2075 card (Windows) when paired with a Quadro card as part of an NVIDIA Maximus™ configuration
Quadro FX 3700M (Windows)
Quadro FX 3800 (Windows)
Quadro FX 3800M (Windows)
Quadro FX 4800 (Windows and Mac OS)
Quadro FX 5800 (Windows)
Quadro 2000 (Windows)
Quadro 2000D (Windows)
Quadro 2000M (Windows)
Quadro 3000M (Windows)
Quadro 4000 (Windows and Mac OS)
Quadro 4000M (Windows)
Quadro 5000 (Windows)
Quadro 5000M (Windows)
Quadro 5010M (Windows)
Quadro 6000 (Windows)
Quadro CX (Windows)
Tesla C2075** (Windows)
AMD Radeon HD 6750M (only on MacBook Pro computers running OS X Lion (10.7.x) with a minimum of 1GB VRAM)
AMD Radeon HD 6770M (only on MacBook Pro computers running OS X Lion (10.7.x) with a minimum of 1GB VRAM)

Many other nvidia cards will work with a small change to a text file, for the most part they work just as good as supported cards however adobe can't guarantee their performance since they only test cards on the list.

there is simple way, which thanks to adobe we can use/test other cards from Radeon:
Update
on mac 10.8 CS6 Premire app,show content, delete txt file: opencl supported cart.txt