Help how do i fix?

[reachmonkey]

i just bought the game and it runs very slowly and the object and ai disappear leaving just shadows any recomendation of what i chould do to have a better time with the game i was wondering if anyone else had this problem or if was just me and i bought the game on steam

Memory 4 GB 667 MHz DDR2 SDRAM
Software OS X 10.8.3 (12D78)
Graphics ATI Radeon HD 2400 XT 128 MB
iMac 20-inch, Mid 2007