Unfortunately I believe it's one of those lucky to get it to work bugs. It worked for me on my first toon and since then I haven't had any luck (after reloggiing etc) in getting the stone to work.
As I recall, you have to approach it from a certain angle to get it to work properly; I tried it and it did not work (in beta) and then in the course of circling around it, I somehow hit the 'sweet spot'.
I couldn't get the Lady to activate either, and the Lover farther in Stonefalls is also bugged. As SG_Celerrimus said, I believe all the Mundus Stones are bugged.