For posterity: This has changed in the newer versions of power-point. Updated directions can be found at this link, relevant excerpt below:
To rename objects by using the Selection Pane
(On Windows) On the Home tab, in the Editing group, click Select > Selection Pane.(On macOS) On the Home tab, in the Arrange group, click Selection Pane.
Click a name in the list to select the object. Click it a second time to make the name editable. The Selection Pane opens on the right side. It shows a list of all the objects on the current slide.