You can either take the ferry or a cab. The ferry is located directly in front of the straw market (near Senior Frogs). When you exit the port area turn right and walk along the water front, it is right there (no more than 5 min walk).
The ferrys are $3 pp each way and run every 20 or so minutes. They will take you to the Paradise Island dock (the ferry ride is approx 15 min). Once there follow everyone else, across the street of the big bridge and go directly to the other side (again about a 5 min walk). It is easy to find.
If you are unconfortable about timing you can take a cab. They are lined up waiting for passengers right outside of the port. The ride is less than 5 minutes, just up and over the big bridge.
Have fun, it is one of the best dolphin encounters that we have experienced.