By Train (buy the tickets for train and bus at the DB booth in the airport building):
Take the shuttle bus number 635 to Freising train
station. The shuttle bus leaves in front of all terminals and runs
every 20 minutes, 21 hours a day. Tickets cost 2 Euro and can be bought
(cash!) on the bus, too.
(Alternatively you may take a taxi from airport to
Freising train station for approximately 25 Euro). Bus Map
Take the train ("RE" not "ICE") from
Freising train station (called "Freising Bahnhof") to Passau main train station (called "Passau Hauptbahnhof (Hbf)"). Routing Queries for Deutsche Bahn (Deutsche Bahn is the national train company)
Walk to University Campus. Walking Map (Start: Passau main train station, End: University Campus, Building FIM, Innstrasse 33)