Getting to the memorial by bus is possible but difficult: your best bet is to either have your own transport or find people to share a taxi.
Travelling by bus takes about an hour, starting on the 900 bus to Winsen, then switching to the 110 to the memorial; on weekends you take line 100 to Winsen. The tourist office in Celle has a timetable sheet with a sketch map, and you can also find a timetable on the Bergen-Belsen website. Check www.efa.de for bus info.
Driving from Celle, take Hehlentorstrasse north over the Aller River and follow Harburger Strasse north out of the city. This is the B3; continue northwest to the town of Bergen and follow the signs to Belsen.
Call 05051-55 55 or book at www.bettertaxi.de/en. Expect to pay €19 one way from the village of Bergen to the camp or €50 one way from Celle.