I have Win 7 64bit and just got the reg cd key entry from gog and unpacked it ran it and it will not allow me to play lan. I bought 4 games for me and my sons and all of use have different keys from gog and none are working all of us have win 7 64bit. WE followed the directions given and it says it installed the keys but we cannot find them in our registry even after a restart.

Maybe this can help you:
http://www.gog.com/en/forum/arma_series/multiplayer_cd_key/post6

In the meantime, i'll ask GOG for my key and see if i can help you.

OK thanks I'll check that out.

Tried that and says bad serial key now.

I can't even run the game after aplying the regedit file.
I'm getting "bad serial number given in setup".