Installing an add-on product requires two steps:
The method by which add-on products are integrated into a Zope instance is by undergoing a change, and over the course of Plone 3’s lifetime, you’re probably going to have to install products using both the old and new techniques. We’ll cover both here.
The older style of add-on is based on the original Zope product system. In this model, any specially-structured Python module that exists inside the
products directory in your Zope instance is installed as a Zope product when Zope starts. ...