These are the hexagon with the assassin's symbol in them(refer to the key on the map if you're still not sure). I was able to easily find these with the iPad app, but usually they will be in the major cities. There are missions in Havana, Kingston, Nassau, and at Cayman Sound (327,334). For the achievement to unlock you must complete all instances of the Hunt. Which means you must kill a Templar. All of the Hunts have 4 missions to complete them.