This achievement does not refer to the safe houses that are already available. You just have to unlock all of the unavailable safe houses, which are shown on the map as tent icons with a lock over them. The three safe houses to the east of the prison, for example, are not required for this achievement. To unlock a safe house, you just have to go there and kill all the guards around it. Once you do, you'll recieve of a message saying that you've unlocked it. Use the map here to keep track of which ones you've found.