The treeview has its own right-click menu that appears when you right-click on a section. If you want to add your first section heading, or new top-level heading, right click the root of the tome (the top element in the menu) and you'll get the option to add a new subheading of the root (i.e. a top-level heading).
Above: Adding a new top level heading - right click on the root of the tome
If you right-click on existing headings, their will be more options available, as you can insert any type of content.
Above: The full right-click menu is shown if you right click on any existing heading