If you think that you have ADD, and want to go about getting a formal diagnosis (assuming you are correct about having it) contact your college's disablity office/center/point person, and ask what they would recommend. Usually they can point you in the direction of a local professional for assessment, or (rarely) the school will have an assessment program themselves.
Think about what you want to get out of this. Being diagnosed with any sort of disability can stigmatize you, and diagnoses of contraversial illnesses like ADD can cause problems you might prefer to avoid. If you want to know more in order to take a workshop or class on tricks to manage you life, and maximize you ability to deal with paying attention and time management, you might skip the diagnosis.
On the other hand, if you feel you would do significantly better if you were allowed to take exams in a room without others, or additional time on exams, pursue a formal diagnosis.
Some ideas I learned from having a younger brother with a severe form of dyslexia, and some attention issues, are:
1. Get a palm pilot, and use it religiously.