I'm having the exact same problem. Just watched the youtube video and think I found the problem. I never ran ARMA2 from steam, just ARMA2: Operation Arrowhead. Just ran ARMA2 and its doing its first time setup, I'll let you know if this works. Yup it worked, just open both ARMA2 and close out, then ARMA2: Operation Arrowhead and close out, then you should be able to run CO