In front of exit Europaplatz take tram M8 (heading Ahrensfelde/Stadtgrenze) which will drop you very close to the the apartments – get off at the 4th stop ‘Pappelplatz’. If your train ticket shows “city-ticket” no further ticket needed. If not buy a short range ticket (Kurzstrecke) at the ticket machine inside the tram (2,20 €).
A taxi from BER airport costs about 70€. In the usual morning traffic jam it takes much longer than the train.