Located in Dunwich Building's exit of the Virulent Underchamber, in the center of a room near the second exit to Dunwich Building. You must go through the other sectors of the building in order to gain access to the Virulent Underchamber, then take the set of stairs above the final doorway from the room with the obelisk. The bobblehead is located in the center of the room on the floor. After that, you may proceed to the entrance of Dunwich Building and fall through the broken floor and exit the building.
This becomes quite difficult if you haven't been to this location before the Broken Steel add-on as there will be numerous Glowing Ones and Feral Ghoul Reavers. Although, the Ghoul Mask can make this bobblehead relatively easy to acquire as you do not need to worry of any enemies.
Alternatively instead of going through the whole building, you can go to where the hole is in the ceiling at the end, (near the door with the bobblehead) and stack up 5mm Rounds to make stairs leading up. At least 7 rounds high is needed to achieve this. It would be easier to stack barrels, but seeing as there are none, you can't.
Obtaining Bobblehead Melee Weapons