Running the demo scenes has been greatly simplified! Navigate to Window > Inventool > Demo Launcher and follow the instructions there.
Using As Starting Point
If you're interested in using a demo scene as a starting point, create a copy of it rather than editing the demo scene itself. This is because Inventool updates will overwrite your changes if the original scene is used.
All demo scenes exist in the Assets/StylishEsper/Inventool/Examples/Demos folder. Go into either the Modern3D, Minimal3D, or Stylized2D folders and copy the scene there and paste it into the location of your choice outside the Inventool folder. At this point, you can safely edit the copied scene.