Use the following train lines from Hauptbahnhof:
S5 in the direction of Strausberg Nord
S7 in the direction of Ahrensfelde
S75 in the direction of Wartenberg
Disembark at the train stop at Hackescher Markt, head out of the Burgstraße exit towards Museumsinsel and walk until you reach Anna-Louisa-Karsch-Straße (approx. 300m on foot)
Take the TXL bus for Hauptbahnhof from Tegel airport, use the following train lines from Hauptbahnhof: As above.
Use the following train lines from Berlin Schönefeld airport (via Berlin Ostkreuz):
Take the S9 towards Pankow. At Berlin Ostkreuz, change to the S7 heading towards Potsdam Hauptbahnhof and leave the train at Hackescher Markt.