You have DayZ commander installed? http://www.dayzcommander.com/ When you have installed arma 2 oa run it up and play a mission or whatever, then close the game, open dayz commander and open install/update tab and check and make sure everything is updated/ installed, I usually download dayz mod from there instead of steam, personal preference, then you can find servers from that as well.
@Kyran Use DayZ Launcher Commander is acid now and not used by much of the community http://dayzlauncher.com/