How do I install The Stanley Parable?
Assuming you installed Steam in the default location, you'll want to unzip the folder into
So, in the case of The Stanley Parable, you'll have a folder
After this, you'll have to restart Steam completely, and the mod will be in the list.
Make sure you own the Source SDK Base (which comes with almost every non-free Source game) and have it installed. (If you don't, it should install automatically when you try to launch the mod.)
You can install and run it through Desura. That's what the "Install Now" link at: