With one of more database sessions, start the Diagram Editor.
From the Tools toolbar button, select the Attach to Session menu option.
The Tools button
If you have multiple session open, the Select Session dialog will open.
The Select Session dialog
The dialog allows you to select a single session to attach to.
One confirmed, the Diagram Editor will be part of the session. If you close the session, the Diagram Editor will be closed as well.
The lists of Tables, Views and Domains in the Diagram Explorer will be populated with the objects from the database so that they can be used in the diagram.
Part of the Diagram Explorer
You can drag and use items from the Diagram Explorer onto the Workspace and use them.
See also: Reverse Engineering