You should disclose your mental illness as the above posters stated. What you want to do in your addendum, however, is be sure it focuses more on how you've been able to persevere and overcome the obstacles your illness presented rather than use it as the excuse for your low gpa. Your first year of law school will be very stressful and admission committees want to know that you will be able to manage that stress during your first year.
My school makes testing accomodations for people with ADHD and may make accomodations for your disorder if it ever becomes unmanageable while in law school. Finally, if you do get accepted, find out if the law school has access to a psychologist or psychiatrist because little stressors can cause even the most mentally stable people to lose focus their first year. Good luck!