If there's nothing on your record, don't mention it, period. If admissions committee members even saw something like that on your record, you can consider your chances to attending graduate school nearly cut down by 99%. This is my personal opinion, but from what I've seen around the forums, cheating is seen as a pretty serious offense.
Also, if you were to try and explain the situation through PharmCas and I were an AdCom member, I'd definitely call the school to verify. Keep in mind if this actually does end up on your record.