I agree with Gerry. The term "chemical imbalance" is a necessary simplification for the layperson of highly complex neuroscientific processes.
Breakthroughs in brain-imaging techniques, however, are helping us to understand conditions such as ADHD as well as the medical treatments that target key neurotransmitters and thus help to alleviate symptoms. For example, this wrap-up of news stories that explain in layperson's terms recent research findings:
http://tinyurl.com/yk47z64
This blog post links to an interview with Dr. Nora Volkow, a preeminent neuroscientist and chief of NIDA, explaining how ADHD is affected by "low dopamine" (again, a complex process reduced to a simple concept, in order to bridge understanding to non-scientists)
http://tinyurl.com/28g85bm