I am doing a debate and arguing that ADHD/ADD is a real disorder and is not overdiagnosed. I need help with references that support this.

Just to clarify, many researchers assert that ADHD is overdiagnosed while still being a real psychological disorder. There is evidence of overdiagnosis associated with limited assessment, as well as conflicting relationships between medical and psychiatric professionals and pharmaceutical companies. Cognitive biases of diagnosticians translate into overdiagnosis of males, particularly of Caucasian descent. It is important to recognize that there are factors that indicate overdiagnosis, but this does not rule out the existence of the disorder.
According to UCLA, ADHD is most commonly diagnosed in children and adolescents. The Center for Disease Control and Prevention (CDC) report a steady increase in prevalence, from 6.9% in 1997 to 9.5% in 2007. Based on scientific evidence pointing to the disorder, it can be argued that part of the increased prevalence is associated with better diagnostic practices and awareness of the disorder.
The DSM-5 (Diagnostic and Statistical Manual of Mental Disorders) intends to reduce clinician subjectivity and diagnostic reliability; it provides more behavioral symptoms, ones that are specific to developmental stages. The DSM-5 also stresses that, despite many of the behavioral symptoms associated with ADHD being commonplace, this does not mean that the disorder is nonexistent. The important element here is the accumulation of symptoms which impact personal relationships, daily functions, academic success, and work requirements.
There is considerable evidence pointing to the neurological basis of ADHD. The disorder is associated with structural abnormalities in the brain, such as lower gray matter density (cell bodies) and abnormalities in white matter (nerve fibers); reduced total brain volume and comparative volumes of certain brain structures; and differences in the cerebral cortex such as delayed cortical maturation and reduced cortical thickness. This evidence is indicated by magnetic resonance imaging (MRI) studies and The Virtual Brain (TVB), a neuroinformatics application.
There are also functional differences that can be seen in subjects diagnosed with ADHD. Regions of the brain affected by the disorder involve the frontal cortex, which supports attention and executive function. Functional neuroimaging studies identify regions of the brain that are under-active or over-active compared to children and adults without ADHD.
Neurotransmitter abnormalities are also present in subjects diagnosed with ADHD. These involve alterations in both activity and communication between different brain regions. Evidence is indicated by delayed maturation of dopaminergic neural pathways in children and adolescents, as well as lower levels of dopamine overall. Additionally, norepinephrine is also associated with attention and focus. Many pharmaceutical treatments address abnormalities in dopamine and norepinephrine, such as Methylphenidate (Ritalin) and Adderall.
http://smhp.psych.ucla.edu/pdfdocs/overdiag.pdf