first copy and paste your mongen.txt file to c:
then press start and choose run in the box type cmd
a dos window pops up it should say c:\
if not type cd\
when it says c:\ you should be in the place where you copied your mongen.txt file
now all you do is type rename mongen.txt mongen.mst and voila you have your mongen.mst file .
now all you have to do is place it back in your mir200\envir folder...
i have tried renaming a txt file to mst format inside windows but i found the way i have shown to be a lot easier...