Give him a break. I had a pretty rush time my self trying to figure that out. So here is what you will do:
1-Create a new sector outside the main map; that is named a control sector.
2-Change one of it's linedefs to "FOF(Floor over Floor), Solid, Shadowcasting. Put him a tag of [some number, specially a number that is not in use].
3-Change the control sector's ceiling height to be the height of the topmost part of your FOF.
4-Change the control sector's floor height to be the height of the bottomost part of your FOF.
5-Change the afected sector (the one that's really going to appear on the map) tag to a tag of [the number mencioned above, also in these brackets].
Simple.