Hi Boxturt,
You can quite easily edit the source code and add a back button on top, where you see the other navigation items 'sign the guestbook' and 'administration'. These you can find in page body.php.
Suggestion: add code <a href="java script:history.go(-1)">back</a>.
But if you're using frames, why not load the guestbook in the content frame, and keep the navigation in the navigation frame? (example: Fairbrook House> choose guestbook)