Starting at the Mourningwood Fort, head straight down the main path towards the forest settlement (You know, where all the eco-warriors live?) and keep an eye to your left. Don't go left into to the Dark Sanctum (Temple of Evil with a fiery gate), don't go to the Ossuary (Where the ghost brothers hang out), and keep looking to the left. You'll eventually come across a path that leads to the place you're looking for.
Just head straight down the main path, keep looking left. Can't miss it.