All existing diagrams are listed in the Navigation pane. To open one of these diagrams, simply click on the name of the diagram. The diagram will open in its own tab in the diagram pane to the right.
The number of diagrams which can be open at one time is set to 5 by default. This number can be changed in the Appearance tab of the Settings dialog. Unchecking the 'max number of tabs' box removes any limits to the number of tabs.