Anyway, I rent there so often that is almost second home to me, I think I've used all the cars there at least 2 times already.
There is no aisle, "pick your own car" service, it is pre-assigned. Typically once I land as I'm getting out of the plane I call 787 791 1805 and advise them of my arrival (you probably will have check luggage, if so, do it once you get them) tell them you are emerald or executive and that should do it. Once you board the shuttle advise your name and status to the driver and he will radio in that he has picked you up. Most of the time you will be dropped right by the car and a National Rep will be waiting for you to sign the documents. You will be asked where you are staying and what phone # to use for contact purpose only.
The main reason why I call National is that unfortunately, they only have 2 shuttles going, one for departure, the other arrival, and I want to speed things up. The bigger company's, Hertz, Avis, have several. I've actually witnessed 4 Hertz shuttles that passed me while I waited for the National/Alamo bus.