@DTLow response is correct (i.e. accessing the "Notebooks" functionality and right-clicking on a notebook to expose the menu) and works in the Mac app. However, the interface is not intuitive and the sidebar must be minimized (not hidden or expanded) in order to access the notebook functionality (and only by left-clicking on a notebook does the "Notebooks" functionality appear, right-clicking displays a menu that does not include "Add to stack...").
This is menu when right-clicking on an expanded sidebar notebook:
This is the menu when right-clicking on a minimized sid