When you arrive at the Radisson, you're in the "Motor Lobby". You need to take an elevator to either the "Ballroom" on the sixth floor or the "Lobby" on the seventh floor. There are two additional buttons on the elevator that are commented out. There are also some surprising colored windows that hint at a much larger room behind.
If you go the Lobby floor and take the two crash doors to the right, just before the bar, you'll find yourself on an open staircase going to the Ballroom floor. It hangs over this giant atrium that's been abandoned. Apparently, the whole place was a mall in the 70's when all of Latin America used to go to Miami to shop and party.
That's the black elevator shafts on the far right. From the outside, you can see they painted the car windows and that at one time, you could see into the atrium from the elevator.
By the way, for some reason, you can only go from the Lobby to the Ballroom, not the other way around.