If you haven't yet, you need to obtain the Decompressive Isotopic Estrangement device (or D.I.E. in short).
When a Megaton spawns, kill it and subsequently kill the two halves. One of them will drop a Keycard which you need to pick up.
Now head to the Weapons Lab in the underground facility and face the cabinet with the broken glass. Use your keycard on the computer next to it and the cabinet will spawn the D.I.E. Remote Control.
Now go to the "Living Room" in the Nacht der Untoten bunker, and notice a crack in the wall with blue electricity behind it. The room behind this crack is inaccesible yet, but you have to interact with the crack which will activate the D.I.E. device behind it, using the remote control.
Now start to kill zombies in front of the machine, You need to kill approximately 30 of them, and every 'succesfull' kill will release a blue Lifeforce from the zombie's body, siphoned into the machine. Once you have killed enough zombies, discharge the device and watch as the door to the room is blown off. Inside the room, you will find a skeleton holding the D.I.E. device. You can now pick it up.