Someone (I believe it was GeneralBarron) once expained to me that it would be possible to test MP missions on one computer, with the server.exe. If I remember it correctly I should download the server.exe, start it up, start the game and then join my own server as a client (even though I'm actually playing on the same computer).
I've never tried it though. Is it possible, and how do I do it in ArmA2? I suppose the server needs to be configured in some way, and I haven't gor a clue how to do that. Any help (and preferably a walkthrough) is welcome.