im not sure, but to make a description .ext file, you dont have to do anything fancy, create a normal wordpad (.txt) file in the same directory, copy that stuff down into it, then save and exit, then at the top of the folder go to tools > folder options > view > show file extensions
then simply make sure that you rename your .txt file
"description.ext" without the quotes of course....
i wudent try editing the mission.sqm file because all the data it holds is relevant, ie: every unit in the game, and the description just holds values which you can change to...