LEGO Pirates of the Caribbean: The Video Game

You get this after collecting all members of the Black Pearl crew. 8 of the characters (Koehler, Twigg, Mallot, Scratch, Bo'sun, Clubba, Grapple and Jacoby) will become available after completing the "Curse Of The Black Pearl" story. You'll find them wandering around the hub, just smack them a few times and once a B appears above their head, you'll be able to purchase them.

Pintel and Ragetti are unlocked during the story in level 2 of "Dead Man's Chest".

Captain Barbossa is unlocked through story progression in chapter 1 of "At World's End. Ding! Achievement!