Ok hidden is relatively easy to get set up, steps below, good luck:

1. First things first, you need source sdk base 2006:
a. hover over the library tab, and select "TOOLS" from the drop down.
b. scroll down and find "Source SDK Base 2006"
c. double click, download and install "Source SDK Base 2006"

2. You'll now need the hidden game type so go to this site and download any of the client "Beta 4b Client Installer"s
http://www.hidden-source.com/downloads.htm

3. COMPLETELY close down steam, make sure the steam icon is not in ur hotkeys next to the clock.

4. Install the Client Installer

5. Once the installer is done, open up steam, and the hidden gametype should be in your game library

6. Watch ur back ;)

Any questions or problems should be directed at me :)