Once inside the Lair of Infinite Agony after coming the main story for the DLC. There is a chance he will appear. When you do see the exclamation point then make your way North through the Seer's Sanctum through to the Hall of the Dead. From there just make your way easily over to Mr. Miz. Talk with him to start the quest: The Amulet, which allows you to buy the Mysterious Amulet or punch him.