To Bad Malente-Gremsmühlen station, then a five-minute walk or taxi ride (book in advance!). The train from Berlin takes 3.5 hours and from Hamburg 1.5 hours.
A1 motorway to Eutin exit, from there around a 20-minute drive to Malente.
To Hamburg Airport, then with a hired car (around 1.5 hours to Malente). Or with the city railway (S-Bahn) to Hamburg Hauptbahnhof (Central Station) and from there by train, changing in Lübeck for the local Malente line (around 2 hours).