Explore your MotS Cd and find the file called "UNSUPPORTED.ZIP" copy to your hard drive and unzip.
Put the files it contained either in a new folder under episodes in your MotS game directory, or you can goo them up with Conman (obtainable from many sites, including www.code-alliance.com
), but be sure you write an "episode.jk" file (use notepad, it's just text).
If you want examples, you can ungoo any addon SP level for Mots and look at their episode.jk file for reference in writing your's (don't use an MP level as a template, or the level will crash when you beat it).
You used to be able to visit the official LEC site and on the MotS page there was a link to a pre-made one, but I don't know if it's still there, been ages.
There are some pre-made versions of the level on the 'net, but they are usually monkeyed with or buggy, because people either didn't know what they were doing, or tried to add new stuff to the level (that IMHO usually didn't work too well).
Once installed, fire up MotS and select "single player" and you should have a new mission to choose (depending on what you named it), select it and go. If the level crashes, it's because you goofed up in the episode.jk file (the level should end, not look for the next level).
A goo/gob program like Conman isn't *required* it's just nice if you like to be tidy.