[Zope] Renderable

I downloaded the Renderable Product today. I admit I'm a newbie to Zope, but every time I try to add an external method for function Renderable in module Renderable I get an error: function Renderable not found in module Renderable. I can add an external method for other python modules, so I've installed it in the right directory. Besides, it's at least finding the module, so that's telling me it's in the right place. What am I doing wrong? It must be something so obvious... Thanks in advance, Liz Pelletier