Are you driving?
Trunk Bay is on St. John, so you'd first have to get to Red Hook and take the ferry over to Cruz Bay. From there, it's about 10-15 minutes out to Trunk Bay. This can all be done via cabs pretty easily. Try to be off the ship as early as possible so you can catch the 9:00am ferry at Red Hook. You can usually arrange times for the cab driver to come get you.
If you're driving, turn left out of the Havensight parking lot, then right at Rt. 313. Right again on Rt. 38, near the hospital, then follow 38 to Rt. 32. Turn right onto 32 and it runs into Red Hook. You have to bear left at Rt. 30 to stay on 32, then left again when 317 goes out to the right.
On St. John, follow Rt. 20 out of Cruz Bay to the north, and Trunk Bay is only a mile or two as the crow flies, but the road twists and turns a bit, so it will be longer on you odometer.