I had the same problem and it took me for ever to find out a solution... everytime i tryed to join a game it said bad version connection rejected server. Then every time i tried to update arma2 it wouldnt update so thats when i figured out that all i had to do to get it to work on dayz commander was to check the box that says "replace orginal arma2 OA files with beta so steam works" and it worked.