I just got Steam for Mac, and it's excellent. I downloaded Portal for free, and installed some custom maps. Here's how it's done.
- Download the maps. You can get them from Portal Game Maps or My Aperture Labs.
- Unzip them.
- Find your maps directory. It should be in Documents, Steam Content, [your username], portal,portal, maps.
- Put the unzipped files into this directory.
- Start up Portal.
- Go to Options, Keyboard, Advanced, and check "Enable developer console." Click OK until you're back at the main menu.
- Press the tilde key (~) and type
map [the map's name].
After a bit of loading, you'll be playing a custom map! Enjoy.
Note: I did not have luck going into the "Bonus Maps" menu. If you do, great success!