f you just bought the bindings, they'll send you base plates for the other mounting options. If they are an older pair of bindings, you can try talking to Burton to get them to send you a 4x4 disc. Aaaand if neither of those are the case, chances are you will have to buy them.
Oh and that link you have didn't work for me. don't know if that's the case for everyone but just in case.