After it's finished downloading open game_mod.rar and extract everything into your Call of Duty Black Ops folder. This is usually located in your SteamLibrary\steamapps\common\Call of Duty Black Ops. In Steam you can also find where it is installed by going to your Library, right click on Call of Duty: Black Ops, go to manage and click "Browse local files".
Make a Folder Named "mods"
In your BO1 folder create a nother folder and name it "mods". This is where your custom maps/mods will be installed.
Once the map is finished downloading Extract the folder to the mods folder you created earlier.
Start BO1 using BO_Mods.bat
In order to play custom maps or use mods, you must launch BO1 using BO-Mods.bat located in the Call of Duty Black Ops folder (Note: Steam must be running before you launch the game) Do not launch Black Ops using Steam or the BlackOps.exe. The map will only work when you launch the game using BO_Mods.bat.
Loading The Map
Once you launch the game using the BO_Mods.bat in your menu you will now have a "MODS" category. Click that and select the custom map you want to play. Once it loads click on Zombies, choose Custom Zombies and click on the map name to start playing.