Instructions
Download the Zombie Plague Mod. Since the mod is created by the game's players, it is available from the Allied Modders' forums. The mod is free to download and will come in a compressed file.
Extract the compressed files and move all the files to the game's "Mod" folder. The folder is located inside of the "Counter-Strike" folder, which is located inside of the "Steam" folder.
Start "Counter-Strike 1.6" by inserting the game's disc or booting the game through its online platform, "Steam."
Click "Find Matches" from the main menu. A window will appear.
Click "Properties" at the bottom left of the screen.
Type "zm" in the map name field. All maps which begin with the letters "zm," which are all the zombie maps, will be displayed. Press "OK" to close the "Properties" window.
Double-click the map you would like to play. Your computer will instantly download the map and join the match in progress.