Instructions
Initialize the Numbers mission from the mission selection screen in "Call of Duty: Black Ops" and wait for the cut scenes to finish. Shoot or throw grenades at the six white canisters on the walls to destroy them.
Walk past the ladder that the characters in the game tell you to climb into the small room. Inside the room will be a cassette player with a black tape sticking out of it. Stand directly in front of the tape and press the "Action" button on your controller. The tape will now disappear or your screen will shake.
Follow the in-game instructions until you get to the weapons cache hidden behind the refrigerator. On the left side of the hidden room will be another cassette player with a black tape sticking out of it. Press the "Action" button again to activate it. Music will now play and the Thundergun will appear above the cassette player. Pick it up and begin using it.