The Overview editor displays the contents of the message set object that you select in the Outline view, and you can view and change this object's most common properties.
To edit any of these values, click the appropriate field in the Overview editor. Depending on the type of field, this either displays a list from which you can choose, or it enables the field for direct editing.
To add a new message model object, right-click Structure and then click the appropriate item from the displayed menu.
For example, the menu for Messages provides Add Message, Add Message From Global Element, or Add Message From Global Type.