1.) First of all, create a new page at "Pages" => "Create page".
2.) Open the page at "Pages" => "Manage pages" and click on the button "Insert Extra" above the editor.
3.) Look for the extra "Guestbook" in the select field and approve your selection by clicking on "OK".
4.) Your addon "Guestbook" has now been added to your page and all you need to do is saving your page by clicking on "Save".
You can manage / edit the settings of your guestbook at "Addons" => "Guestbook". There you can adjust the settings for your guestbooks as well as managing the guestbook entries of your visitors.
Tip: You can add most of the addons to your website the way described above: Contact form, Blog, Link list, Shoutbox, Galleries, Pinboard, Polls, Sitemap, Counter, Member area, Calendar.