Model element inclusion is simply the edition of generation roots. This takes place in the third tab of the document edition window, as well as in the third page of the document creation wizard.
Key:
Note 1: Rather than using the toolbar to add a new model element to generate, you can drag & drop one from Modelio’s explorers.
Note 2: The generated document structure diagram simply enables you to see which model elements are going to be used as document generation roots.