Step 1: Close steam, and launch it again as Adminstrator. Step 2: Right click on Arma 2 in your steam library, then click on Properties, after that click on the Local Files tab, the click Verify integrity of game cache. Then wait for that to finish Step 3: Repeat Step 2 but instead of Arma 2 Verify the Arma 2: Operation Arrowhead. Step 4: After you have gotten that done, launch Arma 2, close it, then launch AO, then close it. Step 5: Now launch Sixlauncher or whatever you use, and you should be able to connect! If you get the message ""1 files failed to validate and will be reacquired"" Then steam found the problem and is fixing the file.