Fly into Hamburg airport, take the city train "S1" (it's easy to find, it's the only one) and after around 30 minutes get off at Landungsbrücken.
From Hamburg Central Station take the city trains S1 (the green line,
direction Wedel or Blankenese) or S3 (the purple line, direction
Pinneberg). After around 5 minutes get off at Landungsbrücken.
Those driving themselves: If you are lucky you my find a place to park your car at the Seewartenstraße or surrounding streets.