When you start the underground survival level, go to the right and there is a tunnel, go through it, look to the right and then you should see a big white truck by the jazz club.

Go on the other side of it and above you there is a balcony. Go to the right and there is a green bench.

Go on the right side and there should be a wall with vines, stand next to it then just run forward and jump.

Then you should be on an invisible ladder. Watch out for Spitters and Smokers though!