adding an edge to a polygon mass object

[Pshupe]

I'm doing a tutorial from the Mastering Autodesk Revit book and it's not allowing me to add an edge to triangulate a surface in a concenptual mass object.  The tutorial says to select the face and add and edge to triangulate, but when I select the face, which only selects two parallel faces it only gives me the option of placing a parallel edge.  This would only split the faces.  I cannot select vertices or "nearest" along any of the edges.  Please advise.  Thanks.

Regards Peter.

[Pshupe]

Here is a screen shot, or what I "cannot" seem to do.  See attached.  I cannot remove the curvature and replace with a triangulation as shown in this example.  It only seems to allow me to draw a vertical edge.  It gives me a "do not" symbol when hover over the points where I would want to draw an edge between.  Please advise.

Regards Peter.