The room number using the official University of Essen campus routing system is

Room T03 R03 D44.

Take the entrance T03 (see picture) or (even better) entrance S03 then take the red elevator (R) to the 3rd floor (03).

Enter the hall with letter D and look for room number T03 R03 D44 which is nearly at the end of the hallway.

For more maps look at

http://www.uni-due.de/imperia/md/images/universitaet/lageplaene/druckversion_standorte_essen.jpg