All the versions are included on the same disc (or, in this case, the ISO file). What version you install depends on what key you bought. Of course, the beta keys are valid for any version of Vista RC1/2 so it really doesn't make sense to install a version other that Ultimate.
on the screen in which you enter the product key just simple press next without entering the key it will then show the versions to choose from which you can then install. All version are on the same DVD (ISO image)