The term MMR:DE refers to a specific Lode Runner project, currently in active development. It's an unofficial built-from-scratch Mad Monks' Revenge (or Lode Runner On-Line) project by QuarkRobot, found here: https://mmr.quarkrobot.com/. The link is also in the Resources tab.

MMR:DE is probably the easiest way to play the game without downloading emulators or abandonware. MMR itself contains the single player levels of TLR, plus additional levels exclusive to MMR, but does not contain the TLR exclusive Co-op levels.

MMR:DE is only allowed for Single Player runs of Lode Runner: The Legend Returns for the above reason. Be sure to mark "DE" for the "MMR:DE?" option when submitting a run.