Game Unavailable

From SteamFriends Wiki

Help! I'm getting a "This game is currently unavalable." error.
There are a few possabilities. One is that Steam might be particularly swamped at that moment, and you should try again in a few minutes. However, it's far more likely that you are missing a few files that Steam deems vital to running the program. You can get said files from the Microsoft Visual C++ 2005 SP1 Redistributable Package.

Grab it from below and the game will work:
Microsoft Visual C++ 2005 SP1 Redistributable Package (x86)
Microsoft Visual C++ 2005 SP1 Redistributable Package (x64)

Ofcourse you can also always try to Verify the game cache or defragmenting the game files by right clicking on the game in your Steam Games window.