Is ADD a recessive or dominant trait?

Hmm. Where to begin? First of all, you said:

"is it a dominant trait or recessive trait. In other words do most people have ADD or do a minority of people have ADD?"

This is completely wrong. Whether a trait is dominant or recessive has NOTHING AT ALL to do with how common it is in the population. They're completely independent characteristics. Polydactyly is a dominant trait, causing extra fingers and toes, yet hardly anyone has it.

As for ADD, well, you're screwed there. The genetics behind ADD is still a matter of intense and undecided research. It's clear that there is some genetic component, but it's way WAY more complicated than a simple dominant/recessive relationship. It seems very likely that there are at least a handful of different genes involved, each with multiple alleles.

Really, at this stage, if your project is supposed to be on genetic disorders, ADD is a very very poor choice. The genetics behind it are very complicated and very poorly understood, and it's also likely to not be entirely genetic. Instead, environmental factors play a major part as well.

If you want to do a project on an interesting disease that is caused by a simple genetic change, look into hereditary hemochromatosis, which is caused by a recessive mutation. This is a disease where the person's ability to sense iron levels in the blood is impaired, so the body is constantly taking iron in from their food. Over many decades, iron levels build up to the point where it can cause serious problems, like heart and liver cancer. One interesting thing about it is that women almost never get the disease, even if they have the mutation, because they lose so much iron in their blood when they menstruate that it's nearly impossible for their iron levels to build up that much. Another interesting thing is that the disease is treated by simple bloodletting - probably the oldest medical treatment in the world. By taking blood, iron levels are reduced, and the condition can be completely managed.