Go to C:\Program Files (x86)\Steam\steamapps\common\call of duty black ops\players then locate your config.cfg, and add the following 2 lines to the key bindings

bind O "sf_use_ignoreammo 1"
bind P "sv_cheats 1"

then start your game and press P then press O to enable infinite ammo for ZOMBIES!!!!

I tried this but when i pressed P then O ingame nothing happened. my ammo was used up

I think they patched the infinite ammo because i had this working for a while and now it suddenly stopped working...