My take on the question, "Have you ever been convicted of a criminal offense" leads me to read that question as "Have you ever been convicted of a crime." If you plead to possession of marijuana as a violation, than you have never been convicted of a crime. Violations are not crimes. Misdemeanors and felonies are. It is my opinion that you can honestly answer that question as "no." I think the web link from the Legal Action Center can be informative. See page 38 and the following pages.
always state the guilty plead and provide an explaination. It is better to disclose and explain then to be viewed as not completely truthful. Your acknowledgement to wrongdoing and admission shows that you want a chance to improve yourself and be honest eventhough you made a mistake in the past.
Marijuana violations are NOT crimes... they aree 'violations' and do not constitute a criminal conviction... if you are certain this was a violation that you olead guilty to then i would NOT divulge it.
In order to ensure that you plead guilty to a violation i would do one of two things. Go to one police plaza and request a certificate of good citizenship, which will give you a print out of your criminal record OR go to the court you were charged at and aask for a certificate of disposition, this would be much faster route as the certificates are given then and there (and the search through the police department, while much mroe thorough, could take several weeks).