Aspyr (designers for Mac version of MOHAA) have announced there is a bug.
Here's what happens:
When you try to load a custom map, MOHAA creates a ".min" file for that map in your "main/maps/dm" for deathmatch maps, or "main/maps/obj" for objective maps.
For example, if you have despair.pk3 (in your /main folder) and try to load it, MOHAA will create a despair.min file in your /main/maps/dm (dm being that it's a deathmatch map)
Mac sometimes does not load this .min file CORRECTLY (it creates a .min file, but there are errors within it) thus, you have a failed custom map that takes you back to the main menu.
Basically, it's like a car and spark plugs. The car is the map and the .min file are the spark plugs. If you try to fire up the car with bad spark plugs, then the car won't run. Replace the spark plugs, and you're car will run like new (unless it really is a piece of junk)
So here's how to fix it:
Either download the .min file to match your .pk3 file from
http://jump.to/machmohaa
OR
Get a friend who has a PC to install and run the map at least once to generate the correct .min file and have him send it over to you via email. Then, you'll want to delete the file Mac has created for you and replace it with the working .min file in the corresponding /main/maps/dm or /main/maps/obj folders. That should fix your map problem.
Troubleshooting: Occasionally when you do the above, your MOHAA will try to regenerate a Mac copy of the .min file, so simply delete it again and once again, replace the bad .min file with the working one.
If you need to, you can send me the map. I'll download it, run it, and send you the working .min file. If you have any more questions, just email me (check profile)
Good Luck!
Thrillseeker