I copied \Steam\SteamApps\common\arma 2\AddOns into \Steam\SteamApps\common\arma 2 operation arrowhead\Expansion\Addons Saw this on another post and it fixed the problem for me
I have the same problem, except i have both Arma 2 games through steam and i have the launch options at "-nosplash -mod=@dayz -world=empty" and it still gives me the "cannot play/edit" error and boots me out right as im about to join the game