Elements can be added by right-clicking on an element and selecting New Element, which adds the element as a child. The standard tree operations apply to elements, allowing you to change their position using drag and drop or cut, copy and paste from the menus or the keyboard.
Generally when adding a new element, the properties associated with the new element are setup automatically and correctly. For example, when adding a new element to a leaf element, the group type property of the former leaf (now parent) element is automatically changed to sequence from none. The element type is changed to none since it now a grouping element. Sometimes you will need to go back and change these automatically set properties (suppose you want a group type of choice instead).