Bremen Airport is located close to the city center. As a hub of Ryan Air, it offers convenient direct flights to many European destinations, as well as generally good international connections via Frankfurt, Paris, or Amsterdam.
Arriving at the airport, take Tram 6 to Main Station, then a local train in the direction of Bremen-Vegesack, get off at Bremen-Schönebeck for a five minute walk to campus. There is a ticket machine on the tram which takes Euro coins, get a Tarif Zone II ("Preisstufe II") ticket which is valid up to Schönebeck.
It is worth checking for convenient connections to Hamburg or Hannover as well. Both airports have train connections to Bremen within two hours travel time.