I know this is an old post, but the cut plane only works on mep objects when the Enable display by elevation option is checked. You can find this under Options-> MEP Display Control. It is the last item on the page. Once enabled, the cut plane will work its magic. I would look up display by elevation in the help file so you can understand exactly what the cut plane does and how to display duct in the 4 different zones created by using the cut plane.