Installing and Playing BO1 Custom Maps
After it's finished downloading open it and extract everything, in the game_mod.rar, 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 Black Ops is installed by going to your LIBRARY, right click on Call of Duty: Black Ops, going to manage and and click "Browse local files:.
In order to play custom maps you must launch BO1 using BO_Mods.bat located in the Call of Duty Black Ops folder (Note: Steam must be running). Do not launch BlackOps using Steam or the BlackOps.exe. The map will only work using BO_Mods.bat.
Now in the BlackOps menu you will 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.