Drag the unregistered feature class from the Catalog pane to the map. The unregistered feature class is displayed in the Contents pane.
If the map covers the catalog view, right-click the item in the Catalog pane and click 'Add To Current Map'. The unregistered feature class is displayed in the Contents pane.
If the catalog view and map are side by side and are both visible, drag the unregistered feature class to the map. The unregistered feature class is displayed in the Contents pane.