Since version 3, permission to add a post in frontend is set to "registered only" as default. As guestbook is using standard Joomla! category feature, this can be change in Guestbook (Category), where you can set the permission for public too.
Go to your Joomla! administration - Components - Phoca Guestbook - Guestbooks. Add or edit current guestbook. In folder Category Permissions set Post Items for Public to Allowed.