I see you have the black one, but what about your memory, is it standard 512 or it already been upgraded. Might have better performance if you install more memory in matched pair.
Altough one of the post saying that he or she get 15 fps on the 512 mb, a lot of factor affect that like your macbook maybe runs several third party programs that use the memory also use the processor resources. Or your internet connection is also used some of its bandwidth portion on your PC without you knowing it, etc.
Try to consult the blizzard for this problem also, if none of this post is helpful for you, they might have better idea since they the one who compile this for intel mac.
Good Luck.