From Berlin (Südkreuz, Potsdamer Platz, Hauptbahnhof, Jungfernheide, Spandau) take a regional train RE4 towards Rathenow, getting off at Bahnhof Elstal. Take bus 662 from the front of the station in the direction Wustermark / Falkenrehde directly to the main entrance of Designer Outlet Berlin.
Please take a 2 hours Berlin ABC ticket (3,10€). You can use the buses, trams and urban rail and underground trains of all transport companies within the Berlin fare zones (A, B and C), including regional rail transport e.g. Eisenbahnregionalverkehrs (RE4) & Bus 662.