The mission.sqm file is encrypted, you need to decrypt it before the mission editor can read it in properly. Somebody here should have a downloadable file called unpack-sqm.zip. Apparently this will allow you to decrypt the mission.sqm file except for the last few lines. I'm not sure what you need to replace the last 2 lines with, maybe somebody here can answer that too. I haven't tried it myself yet, but I thought you might need this info to get started.