Prince Caspian PS3 game...confused!?

I am to the part where you help lucy find aslan. I couldnt figure out why I was stuck so I read on gamespy the walkthrough. It says when approaching a stream to leap over with the horse by pressing the button shown when you get close. problem is I am not having anything show a button to push to leap. Any tips....or is my game corrupt or something. I purchased it brand new about two weeks ago, cant return it cause i've opened it!!!! Help!!

narnia game sux why did u even waist ur money on that? don't buy movie games. they all sux.