Alright, let's get a few things straight about ADHD (formerly ADD).
First of all, I have it, and have suffered/benefitted from it my entire life.
It DOES exist, regardless of what you believe. It is linked to(quite possibly caused by) a disorder of the frontal lobe, the area where concentration, moods, self-control, and self-awareness are controlled. In people who have ADHD, this area of the brain is slightly smaller, or not fully developed. This means while people with ADD are inhibited in their ability to control themselves and their attention spans, they are usually very creative and brilliant, usually having their own way of looking at the world. It is also a fallacy that people with ADD could pay attention if they just tried harder. It's like telling a blind person to squint harder to see. Because the symptoms of lack of control of their attention are linked to certain physical features of the brain, they cannot just "force" themselves to pay attention. It's also misleading that people with ADD cannot focus on anything. It's not that they can't pay attention, it's that they can't really control it. They can spend an entire day just daydreaming through work, and then come home and superfocus on something they really enjoy. It's sort of all or nothing. It's a disorder of contradictions, not of simply a lack of attention.
People with ADD also seek out and require massive amounts of stimulation for their brains. Whether it be video games, movies, pornography/sex, drugs, alcohol, etc etc. We need to have our minds constantly stimulated to even think clearly. Unfourtinatly, many people who unknowingly have ADD end up drug addicts and give up on themselves. Falling into the trap of believing that they are lazy or stupid, never knowing that they have ADD and just need to take a different approach then everyone else. For example, I live with my girlfriend(who is in law school), who does not have ADD. She studies in complete silence with no distractions. In complete silence, I go nuts. My mind just takes off and doesn't stop. I need to have music or the TV or something going on in the background to concentrate when I study. Also If I've had a rather uneventful day, I'll end up staying up till the early morning playing computer games, listening to/making music, drinking, w/e, just to get that stimulation I need to make my mind happy. Otherwise I just lie in bed for hours with my mind racing non-stop.
But ADD is more then just some kind of attention disorder. Because it is linked to physical abnormalites in the brain, it can cause other effects. For example, I cannot for the life of me stay organized. Everything from my room to the desktop on my computer is in disarray. Well, compared to the average person without ADD is it. Sometimes I will just blurt things out at inappropriate times, not realizing it, which is also another symptom of ADD. What I'm trying to say is that ADD is more then what most people think it is.
If you are interested in learning more about ADD there are a few books written by Edward Hallowell, a psychiatrist that has spent close to a lifetime researching/treating ADD. I read his book Delivered from Distraction and found out that he has a practice within driving distance of Boston. I have actually been to him twice since then and he helped me quite a bit. This guy knows what he is talking about.