In order to gain access to the fort, you first must complete the initial mission aboard the Aquila (The Hard Way) This is the last mission in Sequence 5, so it will actually be during Sequence 6 when you can first attempt this trophy. In addition you must have collected at least one Peg Leg Trinket (which is marked by a goblet icon on the map) and returned it to Peg Leg, who is sat in a chair in front of the pier in the Homestead (where the anchor icon is). Once you have spoken to Peg Leg and he has revealed Fort Wolcott as a location, you can use your ship to reach Fort Wolcott by selecting it from the location map when you speak to any Harbourmaster