Instructions
Download the Zombie Swarm add-on through Allied Modders or another CS 1.6 website.
Unpack the content files of the Zombie Swarm add-on and place them in your "Counter-Strike 1.6" game directory, located on your main hard drive in the "Program Files" folder.
Load Steam, the CS 1.6 game client. Select "Tools". Choose the option "Half-Life dedicated server." This will launch the server configuration tool.
Change the game to "Counter-Strike" and set the game mode to Zombie Swarm. Launch the server to start the zombie match.