Only CMS pages (ie pages in the main menu system) can be "edited", the Registration page is not a CMS page and cannot be edited.

You can create a registration agreement that tells of your privacy policy and is shown on the registration page, you could also create a /privacy.aspx page in the CMS, remove it from the menu and put a link to it in the bottom of your layout.master so it shows on every page in your site. This is what I have done on this site as you see the privacy link at the bottom.

The Registration page can also be customized with additional user profile fields.

Hope it helps,

Joe