To step in you can use the down arrow in the bottom left corner of your timeline. You can also map it to your keyboard using your command palette. Stepping in this way will allow you to see only one layer of the nest at a time which is useful if you need to isolate a specific layer for manipulation(simple nesting.) However, if you need to see it as a composite you can use the alt/opt key with the step in arrow and it will show the nest in expanded form. You can also double click the effected clip in the timeline if you are in effect mode or segment mode and that will reveal the expanded nest. To close or collapse the expanded nest you merely have to double click the bottom most layer of the composite. If you use the alt/opt key with the step in arrow you need to use the alt/opt key with the step out arrow to close it.