To run A2: ACR, you need to have ArmA 2 and ArmA 2 Operation Arrowhead (OA) installed. (The best edition currently available is ArmA2: Combined Operations - which includes both, patched etc).
So make sure you have that, ACR won't run on it's own, it is not standalone. Think of it as DLC.
But you are best asking on an ArmA forum, I don't know who else here plays the ArmA series, but it's been a while since I have.