ADD isn't an excuse, but it's a very valid reason. ADD is the reason many people don't balance their checkbooks, check their mailboxes, listen to voice mail, check email, clean out their cars... There are many behaviors commonly associated with people who have ADD and procrastinating is one of them. Anything that requires focus can be as difficult for an ADDer as moving a mountain. The more complicated your taxes are, the more difficult it is to gather everything together, submit or prepare it and review it. It may not be a biggie to a non-ADDer, but it's a huge feat to an ADDer.
(For the record, I filed mine last week - but I still procrastinated because I could've done it in January).