By Train

Arrival at Airport Berlin Tegel (TXL)
Bus 128 (direction Osloer Straße) to stop Osloer Straße, then subway U8 (direction Hermannstraße) to station Kottbusser Tor.
Walking distance to the Kunstquartier Bethanien / ITI Germany roughly 10 minutes.
Alternative: Bus X9 (direction Zoologischer Garten) to stop Jakob-Kaiser-Platz, then subway U7 (direction Rudow) to stop Gneisenaustraße. From there bus 140 (direction Ostbahnhof) to stop Mariannenplatz.

Arrival at Airport Berlin Schönefeld (SXF)
Take the train RE7 (from Airport Schönefeld / direction Südkreuz) to station Alexanderplatz, then via subway U8 (direction Hermannstraße) to the stop Kottbusser Tor as above.
Alternative: Take the train RE7 (from Airport Schönefeld / direction Südkreuz) to station Ostbahnhof, from there with bus 140 (direction Ostbahnhof) to stop Mariannenplatz or a roughly 10 minutes walk.

Arrival at Berlin central train station
Take the S75 or S7 (direction Wartenberg or Ahrensfelde) to stop Jannowitzbrücke from there subway U8 (direction Hermannstraße) to stop Kottbusser Tor.
Alternative: Take the S7 (direction Ahrensfelde) to the station Ostbahnhof, from there again the bus 140 to stop Mariannenplatz or a walk.