It sounds as if your map line for the swamp might not be drawn correctly. Check your map again and see if you are able to erase the line -- if you can, then it's incorrect and needs tweaking.
From the walkthrough:
Click-hold the pencil on the top middle menhir; pass through the top right menhir and end-release at a location near the swamp.
Adrian will comment when it's correct.
Be sure also that you looking in the right spot for the exit off to the swamp. It'll be the active spot right of the gate with the barbed wire.