How to make a CoD Black Ops type of zombie wave system in unity?

I have been trying to make a wave system like in CoD black ops but so far have gotten to nothing. Can anybody provide me a wave spawning system script or give me a link to a package with wave system scripts that can help me accomplish this? Thankyou

Which bit is causing problems? Is it not a matter of:

  1. Instantiate x enemies from a prefab

  2. Wait a certain amount of time

  3. Goto 1.