After working on my ums map in Scmdraft 2.0 (Beta 0.7.3), I save it as a .scx (I've tried .scm and I had the same problem). Then I put the file into the correct StarCraft folder and start up the game.
I find the map without any problems, I click 'ok' to start the game. The mission Briefings opens up, I click 'start', and then BAM the screens turned to black and a little window comes up (open the attach, its an img of the window):
Read the tutorial, but if my map didn't work, it was almost always because I had a unit placed partially off the map.