Do you mean you don’t see the collections pane at all? If so, it’s just collapsed. Click the menus, View → Layout → Collections Pane.
If you see the pane and just don’t see the subcollections, you can expand them by clicking the triangle next to a collection. You can also click on any collection and type + to expand all collections.
If you see the pane and just don’t see the subcollections, you can expand them by clicking the triangle next to a collection. You can also click on any collection and type + to expand all collections.