I just started to play some custom maps myself, here's all you need to do:
- Open SC2
- Log into battle.net
- Multiplayer button
- Join custom game (something like that)
- Search for your game name and open it up
It will find you partners and download files as needed. Played StarJeweled for about an hour last night, it's pretty fun for a freebie custom map.
I tried the DotA clone last night as well, but I don't think I'll be sticking it out. First dealbreaker is not being able to toggle healthbars (does anyone know how to do this???). I am not going to hold down alt constantly while I'm playing. The fights just feel too flashy/crazy as well. I might be able to rememdy this with some graphic changes, but they are just too chaotic currently. No one out of 4 games would answer a single question I asked either. Also I need to rebind some keys, but only for the custom map if possible.