Steps for installing on Mac OSX, courtesy of Macocha:
1. Extract the mod and place it in: (Home)/Library/Application Support/Steam/SteamApps/sourcemods/
You need to add a symlink to it in your hl2 directory. Like this:
2.a. Open Terminal
2.b. Navigate to the hl2 dir: cd /Users/(your user name)/Library/Application\ Support/Steam/SteamApps/(your steam id)/half-life\ 2/
2.c. Make symlink: ln -s ../../sourcemods/thestanleyparable/ thestanleyparable
Now you can run your game from terminal! Just enter:
3.a. export DYLD_LIBRARY_PATH=/Users/<User name>/Library/Application\ Support/Steam/SteamApps/<steam id>/half-life\ 2/bin:/Applications/Steam.app/Contents/MacOS/osx32
3.b. ulimit -n 2048
3.c. ./hl2_osx -steam -game thestanleyparable
If you want nice shortcut to run the game with simple doubleclick download
http://dl.dropbox.com/u/795175/The Stanley Parable.dmg and place it anywhere.
Starting script assumes you have Steam in /Applications and you have only one steam account. To make it work with specific account (in case you have more than one) you need to manually edit the script. To do so, right click on the script and click Show Package Contents, navigate to Contents/MacOS, open run.sh in textedit and add your username there.
In order to run from terminal you also need to set path to libraries like this:
export DYLD_LIBRARY_PATH=/Users/<User name>/Library/Application\ Support/Steam/SteamApps/<steam id>/half-life\ 2/bin:/Applications/Steam.app/Contents/MacOS/osx32
and increase files limit:
ulimit -n 2048
Then do the ./hl2_osx thing.
Or simply use the launcher.