The terms "Terminal A" and "Terminal B" can be confusing to people who have never been to MCO; they do not refer to two buildings, but to two SIDES of one building, the Landside building.
All planes come into one of four Airside buildings, which are connected to the Landside building by monorail trams.
It takes about 2 minutes to walk from your gate to the tram, about 3-5 minutes to get on the tram and ride to the Landside building, and about 3-5 minutes to walk from the tram (on Level 3) to Baggage Claim (on Level 2). When I fly Southwest, my flight's luggage is sometimes begining to come out of the belt by the time I get to Baggage Claim! Of course, if you are flying a participating airline, you won't have to go to Baggage Claim at all, you can proceed directly to Ground Transportation (Level 1) to find the DME desk.
Once you have your luggage, it takes another 2-3 minutes to walk from Baggage Claim (Level 2) down to Ground Transportation (Level 1).
The terminal building is very long, so you might have another long walk once you get onto Level 1; DME is located at one end of the "A" side of the building. Crossing over from the B side ot the A side is done by means of several hallways spaced through the length of the building; total time from Baggage Claim to DME might be 5 or 6 minutes, depending on how far down the terminal you started.
Below is a diagram of the MCO terminal layout. If I remember correctly, DME is located in the "N" section of Level 1, all the way at the end of the building.
Since you can't zoom up on the above image, you might want to download the following PDF file (it's 3mb so it may take a few minutes), which contains all of the MCO Wayfinding maps. Once you have this file, you can zoom up to read the small text on the maps.
http://www.orlandoairports.net:/goaa/images/diagrams/wayfinding_maps.pdf