The Department of Mathematics is located at Eckerstraße 1, in the 'Institutsviertel' just north of Freiburg's city centre.
Leaving Freiburg central train station, cross the street directly in front of it (Bismarckallee) and follow it to the left (due north, parallel to the railroad tracks). At the following big intersection, keep going north. After having passed the parking lot (on your right), take the footpath to the right. The big building on your left houses the Department of Mathematics. To get to the main entrance, walk a few steps onto Eckerstraße. The walk takes about 7 minutes.
There are three alternative airports convenient to Freiburg, Basel/Mulhouse and Frankfurt Main and Zurich.
Basel/Mulhouse Airport: A regular bus shuttle Freiburger-Reisedienst, operates to and from Freiburg, leaving from the French side of the airport. After collecting your luggage be sure to exit passport control on the French side. The bus stop is located in front of the main entrance for Arrivals. Take the bus (travel time approx. one hour) to Freiburg main station (Hauptbahnhof). You can buy the ticket when boarding the shuttle bus.
Frankfurt/Main Airport: There are two train stations at the airport. One for local regional connections and one for long distance. Proceed to the long distance station, Fernbahnhof, (AirRail Terminal) opposite Terminal 1. InterCityExpress (ICE) trains to Freiburg run hourly during the day and take roughly two and a half hours. German Railway timetable
Also Zurich Airport is about two and a half hours from Freiburg by train. See Swiss Federal Railways from Zurich Airport or again German Railway timetable.