When you first enter the eastern forest, there's 3 main paths: one up north, with the blue ether crystal, one a little south of where you enter, and one way down south at the bottom of the map.

The middle path, the one just a little south, is the one with the secret entrance. it has a skeleton corpse and looks like a dead end at first. The secret entrance is in the bottom right corner of that dead end, mostly hidden behind a black tree. It's almost just below that skeleton-- all the way down and just one square right.

In the picture Aura is looking straight at it