We do not know what causes ADD/ADHD. Hopefully we’ll know soon. If there is something that claims it is the cause check the references.
We do know some things about what causes the symptoms of ADHD.
The brains of some ADD/ADHD subjects show different brain activity under SPECT scans and functional MRI scans but differences in function does not necessarily indicate the disorder. There is some thought that dopamine and other neurotransmitters (catecholamines) play a role in affecting the executive functioning and working memory of ADD/ADHD.