You really need the full version of Photoshop. Photoshop Elements doesn’t allow you to open layer sets, which is how Encore organizes menu elements like buttons. Elements does show the contents of a layer set, and you can edit them to a limited degree, but you can’t use the Layers palette to dissect the contents of the layer set.
If you’re content editing your buttons in Encore, then you may be able to get away with using Elements for other work, like editing the backgrounds. If you’re serious about editing your menus, though, get Photoshop CS (or CS2, when it comes out).