After Joseph has opened up the 2 doors in chapter 6, you will come to a big open space with lots of enemies around. Go right and up the stairs. Climb up the ladder and always look for Joseph. He sometimes stops or gets in a fight if enemies are around. In this case help him out and he should follow you.
Once Joseph has climbed the ladder stay back and wait for him to take on the enemies on the bridge. He will fall off and you have to pull him up again (
).