that's because the game is programmed to run on only one core. you can do nothing to change this (hope for a patch perhaps). the programmer hast to parallize the work (separate calculation of game engine and enemy ai for example) to make use of both cores (or more in future).
sorry i have a lot of questions for you all tonight this is my 5th.
but i have a duo core t2300 1.67ghz, 667fsb, 2mb cache,
when i play ghost recon advanced warfighter on low settings (800x600) and high (1280x800) both on medium then if i go to taskmanager i only see 1 cpu does all the work. why does the other do nothing? can i split games into 2 cpu's?