You are starting to make a name for yourself. Now word has reached you that Captain Kidd himself wants to meet you. Better sail to Tortuga and introduce yourself.
As it says in the mission description: simply sail to Tortuga and talk to William Kidd. He'll be in the Crime Lord's Office - the entrance is a longboat, you have to click on it to be taken there. The NPCs that give you the pirate refit missions are in the same room.