I just fixed it! Thanks for the help! This is how to fix it if you are like me: Problems: I only had Arma2 and Arma 2 OA. The Arma2 OA didn't update to the update I was patching it to. When trying to join the versions mismatched. Solution: Open Steam, download the Arma 2 OA (and Arma 2 and Arma OA also), open DayzCommander -> Settings -> Check Replace original Arma2OA files with Beta so Steam works Thanks very much for the help Aitchy and mZLY!