All the workbenches for this map are right where their markers say they are, but all 3 of them are 1 floor up. Climb up any of the metal ladders, and then make your way to the room with the marker.

If you fall no worries, you can use the nearby ladder to get back up and try again. The second Crystal Skull is found on a bridge that passes over a long stone staircase. The bridge is guarded by ...