...the important thing in that quote... SceneGraphVisitor
As in a thing to visit the scene graph... ie: SceneGraphVisitor
Which you can find in the Javadoc... or even look at Spatial's javadoc and find the method that takes a SceneGraphVisitor.
You can even make your life easier by looking at the already defined implementations of SceneGraphVisitor in the javadoc.
Hopefully you know how to find the javadoc.
In the scene editor, there is a tree. That tree will have your model. If you expand the tree, there must be a geometry in there somewhere... else nothing would be displayed because only Geometry can produce visuals.