LAUNCHER IS BROKE - it's not you, it's everybody
Go in via Steam > Games > ARMA II OA > Multiplayer . . . the old fashioned way
You might get an error about .PBO files. If you do, just MOVE all the .PBO files in the listed directory somewhere else (don't delete them, just in case). I just made an "ARCHIVE" subdirectory right there in the AddOns folder and moved all the .PBO's there.
Make sure you shut down ARMA first or you'll get errors about those files being "in use" - derp