Asking the same Thing.

Alrighty, since then, I had extracted melonbomber from its .gma and checked it out a bit.
of course I decompiled the map to check it out....
...its a empty box.

no randomly placing blocks or anything.. its just empty.

so after that i was checking out some other folders, and found where the REAL levels are....
all the levels are, are .lua files, with a bunch of 'for' statements that place blocks.
thats it.

edit: so creating levels would require disabling melonbomber, and keeping the folder that you had extracted from the gma, making a level, and playing the level after a vote.