The monorails don't go from the A to B sides, they go from the Airside buildings (where the gates are) to the Landside building (where security, ticketing, baggage, transportation, etc are).
Getting from A to B is accomplished on Level 3, immediately after you get off the monorail from your gates. Just follow the signs for DME when you get off the monorail.
AirTran operated out of Airside Bldg #4. The monorail from Airside 4 connects to the B side of the Landside building, so you won't have to crossover to get to DME. Just take the escalator down to Level 1 as soon as you get off the monorail.
Once on Level 1, you will have to walk the entire length of the terminal to get to the DME desk. The terminal is quite long, so it will be a long walk for you once you get to Level 1; you'll pass all of the various rental car desks along the way.
Check out the sticky at the top of this forum,
"A Newbie's Guide to Orlando International Airport." It has directions and maps that will help you understand the layout of the airport better so you won't have any trouble finding your way when you land.