Can I become a pilot if I am under treatment for Attention Deficit Disorder?
I'm going to assume you're asking about US/FAA requirements - If not what's below may not be particularly helpful.

ADD is not a disqualifying condition, however if you have a formal diagnosis of ADD or ADHD you may need to undergo additional testing in order to receive a medical certificate.
Also some of the medications used to treat ADD/ADHD may be disqualifying (I know that Adderall, Focalin, and Ritalin were all on the FAA's "No Fly" list - I'm not sure if they're still an absolute NO or if you can get approval for them).

AOPA has a pretty decent drug list that you can search to see if what you've been prescribed is permitted - it's a little easier to search/cross-reference than the official FAA "Do Not Issue/Do Not Fly" list and they do a good job keeping it up to date, and a brief article on ADD/ADHD &amp; medical certification.
They also have a service to assist you with getting your medical (as part of Pilot Protection Services) which can be helpful.