This article will explain how you can add a new Guestbook message on InstantSite
Scroll down to the Create your website section and click on the InstantSite icon.
Click on the Login button next to the relevant InstantSite package.
Click on the Edit link.
In the Site map panel, click on the Guestbook page.
Click on the Add New Message icon.
Fill in your Guestbook message details and click on the OK button.