To find the map in the Forest Maze, take the bottom exit to the east, then fall down the waterfall. Exit to the west, and there is the map.
To get to the area of the World Map that has the Forest Maze, you need to pass through the cave to the north-west of the mountain town. It requires at least 4 crystals.
5/15/17 - Appears to be glitched. The game crashes when I get close to the map. Trying to reach out to the developers.
9/1/17 - I received an email from PlayEveryWare today saying that they have found the issue, have a fix, and are working on creating a patch to submit for XB1 and PS4.
2/7/18 - Game received a patch today, fixing the achievement.