Here is a map of the DESY campus
Manstoppers should be put already by Friday or by Monday morning across the campus (main gate, building 1...) to show directions to the seminar room.
To go to the seminar room from the main gate or DESY hostel, go to Building 1. Once inside building 1, go along the corridor until 1b. Papers with directions to Seminar Room 4a/b (Seminarraüm 4a/b) should be in place already, go to the second floor. The seminar room is just in front of a small resting room and at the end of a side corridor on the right.
More details here: https://indico.fnal.gov/event/21340/picture/6.pdf