The Humboldt University math department is at Rudower Chaussee 25 in Berlin-Adlershof, and can be reached by public transportation in about 45 minutes to an hour from the center of Berlin. (Alert: although the official address of the university is at Unter den Linden 6 in Berlin-Mitte, that is only the Hauptgebäude (main building); the math department is nowhere near there.)
Unless you're traveling directly from the airport (see below), you need to take the U-/S-Bahn network from wherever you are to the S-Bahn station Adlershof, which is near the southwestern extreme of transit zone B. The BVG website includes a trip planner that can tell you the quickest itinerary to Adlershof from any other station, though it is usually not too hard to figure it out from the transit map, e.g. from the Hauptbahnhof (main railway station), you can get there directly on S-Bahn line S9 (direction toward BER Airport) in about 35 minutes. You will need a ticket for zones A and B (the same ticket you need to go almost anywhere else; note that it works equally well for U-/S-Bahn trains and for buses or trams, and transfers are free).
From the station at Adlershof, you can either walk about 15 minutes southeast along Rudower Chaussee to number 25, or take a tram (any line) or bus (lines 162, 163 or 164) two stops to Magnusstraße, which puts you right in front of our building. Note that when you're on the platform at Adlershof, you will see the street you need running perpendicular to the train tracks in two directions, one of which looks like a residential neighborhood, while the other direction is a mixture of construction sites and unattractive modern-looking buildings; needless to say, the right direction is the unattractive one.
Finally, when you enter the building at Rudower Chaussee 25, you need to understand that it is actually four buildings with a common ground and first floor, and you must choose the right one of the four before you try to go upstairs. For my office, follow the signs to Haus 1 at the far right hand side, then take the elevator or stairs to the third floor; you will find room 301 at one end of the hallway.