1. I had to copy cmd.exe into my MDLC folder to be able to run MDLC with the command line parameters. The model file you want to convert has to be in the MDLC directory as well.
2. Go to the DOS prompt (Start - Run - cmd.exe), switch the c: drive, change directories to C:\MDLC
You are now ready to use MDLC.
If all you are doing is converting aircraft, then I recommend downloading and using Morten Krogstad's MK_MDLC windows utility. Copy the files in the zip file into the MDLC folder, then double-click on the mk_mdlc.exe program file to run it.
Hope this helps.
-James