I found some typos in the code of dm_main.php. this may be the cause of the editor not working.
On several occasions, the author swapped the letters w and n in the words containing "download" in dm_main.php. I did a search and replace and found 4 occasions where the letters were swapped. After fixing, i was able to edit at least some existing downloads, but not all yet. The rest still can't be edited, but i'll try to add these again, with simpler descriptions this time, maybe that will work.