I've only recently tried anything with the level editor myself. The window of the editor is actually taller than my screen, and there is no good way for me to work with the lower options.
I tried to start really easy, making a multiplayer environment, but it doesn't seem to work. It doesn't even seem to work if I start from an existing map, change it to what I want it to look like, and than save it as its own mission.
I was going to try to make smaller changes, to see if it was about anything I messed up, and I was going to try making a normal singleplayer mission, to see if that would work, but apparently it's something more basic, if more people are having this trouble.
EDIT: I've been digging into the forum. Apparently, the editor works for some, but there's no clear reason it doesn't for others. That also seems to be the only old thread I can find about this issue, and it doesn't contain any answers...
It does say something about patch 1.4 being changed after it turned out the documentation wasn't showing in the help of the editor in KOTS (mine doesn't show), but that shouldn't affect the missions playability, right?
EDIT again: O no, wait, there's some good tips here. I haven't done anything with xml files yet, so I'm going to try reading that very carefully. Maybe that could do something.
And @ mjkerner: What exactly does the error message you're getting say? Is there any more info in it?