After talking to Shimwiki he will open up a hatch in the floor for you to enter. Follow the tunnels and climb up a ladder that leasds to a platform in an isolated little lake. Then jump down into the water and swim aroud the edge of the lake until you find a path. Follow the path and you will find it. You can also check Xignal´s maps here
http://www.volberding.com/WHints