- Make sure you downloaded 'client' and not 'server' files.
- The 'client' files will actually be an installer.
- Open the installer and install the game. (It'll set up the directory for you.)
- Open Steam (or reopen it) and the game should be in your list, toward the bottom.
You need Source SDK Base installed as well. This is in your 'tools' tab in Steam. it comes bundled for free with many major Source based games. If you don't have it, then you can't play unfortunately. The cheapest game you can buy that offers the Source SDK Base is Half Life 2 Deathmatch at $4.99