Thanks for the post. There are a small number of features which are available in the app. but which are not exposed in a menu option. In this case, the ‘Tab’ keypress has no effect if you have, for example, a control selected in the canvas - it only works when you have a wireframe selected in the Navigator panel to the side. It’s hard to know how to communicate that in a simple menu option so it’s omitted.
There’s a great overview of the whole editor here (Balsamiq Mockups 3 Application Overview) and that document includes a description of the hierarchy feature.
All this said, thanks so much for the feedback - it’s appreciated.