ADD is no longer a diagnosis in the most current version of the Diagnostic and Statistical Manual of Mental Disorders. ADHD is the current nomenclature and has 3 subtypes, hyperactive-impulsive type, predominately inattentive type, and combined type.
I wasn't diagnosed until recently, at the age of 45, but outwardly my symptoms look different as an adult than they did as a child (and yet nobody recognized it back then!). When I was young, I couldn't sit in a chair without jiggling my foot, for instance. My nerves were always on edge, and I wish I'd realized sooner how much good exercise could have helped. I exercise more and have a job now that has both brain work and physical labor. The physical labor parts are very soothing and satisfying to somebody with ADHD.
If you have trouble concentrating, you need to notice which subjects catch and hold your attention. Math and chemistry put me to sleep, but there are subjects that can hold my attention, and it makes all the difference in learning.