At MCO with Dollar Express, you go out to the garage and they have an electronic board that will have your name and car space number listed. You take that car or have the option to pick another (same class).
Since MCO has side A and side B, you should input your flight info and it SHOULD show up on the proper side. I say "should" because when I went over Marathon weekend, it didn't. I went to side A(since that was the side AirTran baggage was on) and I wasn't listed. It wasn't a big deal because the agent at the booth printed out a copy of my contract and walked me over to the cars. But had there been a line of people there, I would not have been happy to wait.