Ask one of the guys in TeamSpeak to help you out. Most of them have the Steam version, and can probably assist you.
I have the DVD version myself, but from what I hear, you have to install and then run each instance of the game before anything else. ie run Arma 2 =>close the game; run Operation Arrowhead => close it down, run Steam as administrator etc.
One of the guys will be able to give you the full run through with the Steam version.
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