To activate The Pitt DLC, just load up a previous save and wait about one minute or so.
Follow the marker on the map and you will meet up with Werhner and talk about traveling to The Pitt. Follow the map marker and get the slave outfit. After meeting back up with Werhner, follow the map marker to the tunnel entrance and active the hand cart to travel to The Pitt. Follow the path with Werhner. After he opens the gate, continue across the bridge. Follow the road after the bridge and to the right. The gate at the end of the road will be opened for you. After the conversation, enter the door on the left.