When selecting an element in the UML Navigator view, the element is not automatically selected in the Diagram view. To show an element selected from the UML Navigator view in the Diagram view, use the Select on Diagram command. Using this command, you can open the corresponding diagram of the element and highlight the element on it.
To use Select on Diagram:
Right click on the element in the UML Navigator.
From the context menu, choose Select on Diagram.