The forest is located directly at Fangschleuse station, about half an hour from Berlin main station and is easy to reach with the RE1 Berlin-Frankfurt (Oder) train every half hour. Alternatively, you can take the S-Bahn to Erkner and from there the 429 bus.
Attention: The cops often stop people on arrival to get their personal details if they are supposedly wearing masks in order to file charges. Watch out for one another.
Roadmap as PDF.
Location: 52.404969, 13.822031