Is your downloaded DFX a legit one you downloaded from Nova, or a pirated?
Anyways, download this:
http://www.deltaforcegaming.com/chrispy/misc/dfxmed.zip
It has dfxmed.exe and also the DFX MED manual
Put those into your DFX directory and open up the MED to see if it works. If it works, mess around with it and make a map. I also included the manual just incase you didn't have it as well. Read that manual buddy - it helps A LOT.
And what is with people having MED problems with a downloaded version? Did Nova try to disable it or something...?