After returning from the Dark Aether (follow the mission objectives), the Megaton enemy will spawn. Killing it will cause it to split into two. Upon killing both of the split Megaton's, the last one to be killed will drop a keycard. Pick this up, then head into the bunker using the initial entrance. Take the path to the right, and in the far right corner of the room that path leads to will be a cabinet. Interact with it to open it and retrieve the "D.I.E Remote". Head back to the starting area.
Use the door on the left side of the building to enter it. The "D.I.E Machine" will be on the opposite side of the building from the door, and can be seen through a hole in the wall by moving the the right side of the building. Interacting with the machine will cause it to suck nearby zombies into it for a short period of time, before needing to recharge. If enough zombies are pulled into the machine during this period, the remote will indicate that it is ready to fire, at which point interacting with the door in front of the machine will allow you to pick up the D.I.E.M.