[Zope] Product installation

As I understand it, all that is required to install a product in Zope is to extract the archive to the Products directory and restart Zope. This is what I have done in the past and then the product shows up under /Control_Panel/Products. For some reason this does not happen with PloneGuestbook on my installation of Zope 2.7.3 on Linux Redhat v7.3. Can anyone suggest why? Or where should I look to see any signs of problems? No relevant msgs are to be found in the logs. -- John