It's an editor update. Holy f@#%, Batman!
Now I know it has been a while, and I can't even remember how much has changed since the last official "update", but I can tell you that you no longer need that annoying XNA Redistributable package (/spit). I've rewritten the whole graphics part of the application in OpenGL using the Open Toolkit Package for C# bindings. You don't need to know exactly what that is unless you're going to build the code (which I know you aren't!), so all YOU need to know is you can just download and run the program now. That is, unless you haven't updated Windows for years, then you still need a .NET update, but that's your problem, pal.
Anyways, try it out, try and break it, and do tell me if you do. I like it when people try to break it, so fill yourself with rage and have at the editor! And once you've calmed down and returned from anger management, post a comment or a bug report on the issues list.
SMTC Level Editor (Changeset: cb2d164dd0, Build date: 100628)
EDIT: Updated project once more to fix a miscalculation.