Customizing he "Join Membership" page

Hi,

After a user clicks the "Buy Now" button (inserted using shortcode), they are taken to a "Join Membership" page. See here: http://testmybw.prohealthsites.org/register-2/?membership_id=8895

Where can I edit this page? I would like to add some instructions but I don't see it in the list of pages for the site.

Thanks!