You should probably start off by reading the "sticky" at the top of the Transportation board.
There are many websites to provide you the info you need. Here's one that I think explains it all pretty well:
http://www.allearsnet.com/acc/mexpress.htm
And by the way, the participating airlines are American, Continental, Delta (including Song), JetBlue, Northwest, and United (including TED).
Before you read anything, keep in mind that "participating airline" only matters on your RETURN HOME FROM ORLANDO. When you leave home and arrive at Orlando, EVERYONE on ANY commercial airline arriving at MCO gets EXACTLY the same treatment. That bears repeating:
When you arrive at MCO, everyone on every airline uses ALL of DME in EXACTLY the same way. I've read posts where people thought they had to go to MCO's bagagge claim to get their luggage and drag it to the bus and then drag it into their resort -- NOT TRUE!
Oh, did I mention that regardless of airline everyone uses DME in the same way upon arrival at MCO and WDW?....