I am usings the code to produce the attached image.
  view shape: (MOArrowedOrthoVerticalLineShape new).
  view edges: inheritance from: #superclass to: #yourself.
    

This one gave me a hard time. Update Roassal and try the following:
-=-=-=-=-=-=-=-=-=-=-=-=
	view shape rectangle withText.
	view nodes: (ROShape withAllSubclasses). 
	view shape: (ROOrthoVerticalLineShape new add: (ROVerticalArrow new offset: 1 )).
	view edgesFrom: #superclass.
	view treeLayout. 
-=-=-=-=-=-=-=-=-=-=-=-=

In the example I have added many examples about arrowed edges.

  

And just to document something I got working... previously to hide lines while keeping the layout I was using...
  view shape: (MOStraightLineShape new color: Color white).
  view edgesFrom: [ :xx | xx - 1 ].
and now...
  (view edgesFrom: [ :xx | xx - 1 ] ) do: [ :edge | edge - ROEdge ] . 
    

Yep. An example was provided.

Cheers,
Alexandre