1) Start a Private Match on the map "Nuketown 2025".
2) Shoot the head off of every mannequin in ninety seconds or less.
3) The mannequins will be in different places each time so you will have to look around the whole map.
4) If you did this on time go over to the Nuketown sign that shows the population.
5) The TV screen in the middle of the map will show an Activision logo.
6) Interact with the TV screen.
7) You will be able to play any of the four following games: H.E.R.O, Kaboom!, Pitfall and River Raid.
8) While you are interacting with the scree, you cannot be killed.

Make sure you shoot the mannequins outside the map too. There should be about fifteen mannequins.

Here is a video tutorial: