The National desk is on the lower level with the other car rentals - just about as far opposite the luggage carousels as you can be. Once you get through at the desk, it's a short walk out the door, up the escalator and the National lot is just to the right. Lots of signs so you can't miss it.
I belong to National's Emerald Aisle and have always gotten the luggage and then headed down the terminal (toward the National desk but not quite that far), gone out the door, across the street and up the escalator to the National lot. The attendant at the exit gate will check your ID, etc. when you leave. Emerald Aisle definitely makes it easier in Orlando!!! You avoid the line at the desk and are quickly on your way.
Will be there 11/13 for a few days - just made the reservation with National yesterday and got a really good rate (again!) Enjoy!!!!