To add a new element, you right-click with the mouse on any of the following elements:
Top Level: 
to add a ScreenGroup or Screen element
ScreenGroup: 
to add another ScreenGroup, Screen or FieldMap element
Screen: 
to add a FieldMap element
After right-clicking on the element, simply select the type of element you would like to add and click on it:
The above image shows a new Screen element being added to the ScreenGroup named "Welcome"
