The boarding area for Magical Express is on the 1st level of the B side, in the main terminal. If your airline is on the A side (Southwest, Frontier, JetBlue), you will need to cross over to the B side, while still on the 3rd level of the main terminal. That means making a left turn, after passing the Security lines, and taking the elevator (keep walking, till you see elevators) down to the 1st floor. After exiting on the 1st floor, turn left and head all the way to the end, passing all of the car rental counters. Your Magic Bands will check you in, so make sure to either be wearing them or have them handy, for easy check-in. Your confirmation letter will also work, but MBs are easier, IMO. If you need to collect bags, you will still have to cross over on the 3rd level, if you are on the A side, in order to get to the Magical Express area.