Right outside of the arrival building there is an entrance to the subway. There is only one line and it goes only in one direction, so just get on the train. Get off at the “Opernhaus” stop, after about 15 minutes, and go up the stairs in the middle of the platform. When you arrive on the street level, take the path left through the arch in the city wall and turn left again along the inner side of the wall on “Frauentormauer”. The Hostel is the second house on the right, no. 42.