For Multi-player Zombies the only cheat you can use is the infinite ammo cheat.
Open the following file:
C:\Program Files\Steam\steamapps\common\call of duty black ops\players\config.cfg
Start a new line at the end of the file and add these lines.
Save and close the file then right click config.cfg and go to properties.
Make the file read-only by ticking the check box in the attributes section and click OK.
Start a game of Zombies and press the zero key to activate infinite ammunition. To deactivate it you can press the nine key.
You must be the host to be able to activate the cheat, you can see who the host is by looking to see who has the lowest ping.
In this example "C:\" is the drive where Black Ops is installed.
For windows Vista/7 Steam will be located in the "Program Files (x86)" directory.
You can bind the cheat to any keys you wish such as F4 and F5 e.g.