The Physics Bldg. can be recognized by the capital "A", "B", "C" and "D" letters, marking the floors in the building, and well visible from the outside.
Take one of the elevators in the Physics Bldg. to the B level. As you exit, turn right and pass a hallway on the left side. Turn left on the second hallway. The seminar room is on the right hand side, close to the end. The seminars are usually held in room B-131 every Friday at 1:30PM.