In Nacht der Untoten open up the stair case in the main room.

You can go down to the help room if you would like. You need at least 2 people.

One person has to stand right next to the wall where you buy grenades upstairs. The other has to stand in front of a cabinet.

You can find the cabinet by going to the help door, turning around 180 degrees and to your left there is a cabinet like structure.

I recommend the person there has a trench gun because the zombies do come at you rarely.