The shape displayed by the node is specified through its Content property. To create the content, load a Visio stencil file (*.vsx) using an instance of the VisioStencil class, then use the static VisioContent.Create method. A reference to the master shape of the content can be obtained through the Master property.
The standard stencils installed with Visio are copyrighted by Microsoft, and you should not use them in your application. This feature is intended to let you use public domain stencils, or commercial ones you have purchased a license for.