When you go onto Nazi Zombies you should find a door in the main room (hell/help room).

Open that door, then go into the room and you should find the random box (left side corner).

On the box there are gold flashing question marks (?).

You need quite quick reactions to do this - when the question mark (?) turns dark gold press it as quick as you can.

This should get you all the best weapons, e.g. (browning, ray gun).