You can take bus 124 from the train station (Hauptbahnhof) to the university restaurant (Universität Mensa) or to the university bus terminal (Universität Busterminal - this is the final stop).
If you'd like to go from the city to the university, you can take the buses 101, 102, 109 and 111 from Rathaus (city hall) to Universität Mensa or Universität Busterminal: use the Fahrplanauskunft to find the departure time of the next bus.
The bus fee can be paid when entering the bus (you need Euro cash, but not the exact fare). It's 2,50 € for a single ride and 2,10 € with a BahnCard.
A map of campus can be found here. The bus stop Universität Busterminal is the little green-yellow-white H in front of the big parking in front of E3 1.
The mathematics building is E2 4. Our offices are on second and third floor (301-305, 428-430).