2.5 Retopology

I’m using a tutorial book and it mentions retopology but not in detail and I can’t quite understand it. I set up like it says but when I extrude the vertice it doesn’t drop onto the model I sculpted. What’s wrong? The book I’m using is Beginning Blender by Lance Flavell.

Here’s a tutorial showing the process of retopologising in blender 2.5

are you snapping (shift-tab) the vertexes you move/create to the faces of the underlying mesh?