At the moment, the toolbar diagram actions depend upon selection in the Model Explorer tree.

Better to enable the buttons by default.
If selection is made in Model Explorer tree then button uses that as before. If there is no selection then throw up wizard asking the user to select which models to add. So in the case of 'Add Transformation Source(s), all source and view model transformation diagrams nodes will be selectable.

Just helps the user avoid being confused as to why the buttons are disabled. Also helps with the view being the starting point for the use case. Maybe, consider New... buttons that would allow new sources to be created, ie. user starting from top-down approach with the view and working back to their sources.