Take subway U2 to "Ernst-Reuter-Platz".
S-Bahn/Train S9 (to Spandau) or regional train (RE7, RB14 or FEX) to "Zoologischer Garten", then subway U2 to "Ernst-Reuter-Platz".
Alternative: S-Bahn/Train S9 to "Tiergarten" and a 10min walk along Straße des 17. Juni.