When you were creating your guestbook it should have asked you what directory you wanted to save it in on your public_html folder.
If you chose for example guestbook, your link would be
http://www.yourdomain.com/guestbook
If you don't know what directory you saved it under, go into your Cpanel and look in your file manager to find your guestbook. It should be somewhere in your public_html folder.
Good luck, I hope this helped.