self.wizardNotebook = self.xml.get_widget("wizard_notebook")


....


    def on_bttn_next_clicked(self,widget):
        log.debug("Next clicked...")
        self.wizardNotebook.next_page()

        pagenum = self.wizardNotebook.get_current_page()
        widget.set_property( "sensitive", pagenum + 1 != self.wizardNotebook.get_n_pages() ) 

    def on_bttn_back_clicked(self,widget):
        log.debug("Back clicked...")
        self.wizardNotebook.prev_page()

        pagenum = self.wizardNotebook.get_current_page()
        widget.set_property( "sensitive", False == (pagenum - 1) )