Depends on the port you are at
If Langelinie I think the #26 bus will get you there
The HOHO bus was at the dock when we were there
so that might be another option.
Taxi is probably the easiest
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012