If by plane, there are two airports: Berlin Tegel (TXL) and Berlin Schönefeld (SFX).

From TXL airport: bus station at the ariport, bus X9 or 109 in direction S+U Zoologischer Garten (3 stops) -> metro station U Jakob-Kaiser-Platz metro line U7 in direction Rudow (8stops) -> metro station U Fehrbelliner Platz metro line U3 in direction Krumme Lanke (5 stops) -> metro station U Dahlem Dorf. From Dahlem Dorf you reach easily by walk.

From SXF airport: city train station close to airport S Flughafen Berlin Schoenefeld, train S45 in direction S+U Bundesplatz (14 stops)-> S+U Bundesplatz metro line U9 in direction S+U Rathaus Steglitz (4 stops) -> from S+U Rathaus Steglitz short walk to bus station Albrechtstr., take the bus X83 in direction Koenigin-Luise-Str./Clayallee and get at Museen Dahlem Berlin. From Museen Dahlem you reach easily by walk.