I have a friend with ADD who has worked for two airlines. ADD usually improves with age.
However...the answer to your question is a little more complicated than yes/no.
As an airline pilot you have two seperate but related medical certification issues...
1) FAA Medical. All pilots need one, airline pilots (and most other professionals) need a first class. Don't even worry about or consider a second class medical. You can get an FAA medical with a history of ADD, as long as it is not currently serious enough to create a problem in the cockpit.
2) Airline Medical Requirements. In the past most airlines conducted their own medical exam and history review, usually to much more rigorous standards than the FAA. The good news is that most US airlines are getting away from this (due to cost and employment lawsuits). Some airlines (including most regionals) will simply accept an FAA first class medical, while a few will do a quick exam simply to verify that you meet FAA standards, but nothing more. Expect most foriegn airlines to do a multi-day astronuat physical...which most folks (probably including me) will not pass.
Potential problem with FAA medicals: If you have a disqualifying condition, they might give you a waiver or "special issuance" medical certificate. You do not want one of these, you want a "clean" first class medical...the only acceptable restriction is "Must Wear Corrective Lenses", if you need glasses. Some airlines may actually require a "clean" first class...if the FAA or AME wants to give you an SI or waiver, try to fight that and get a clean medical.
Waiver/SI first class or second class medicals will allow to CONTINUE to work at an airline after you are already employed there...but to get hired your odds will be better with a clean first class.