I had to write an addendum for an alcohol citation that was dismissed strictly due to the rediculousness of the officer writing the citation in the first place (drinking a beer in Times Square on New Years). I did exactly as the above posters wrote: if they asked "have you ever been convicted?" I just checked no. If they asked "have you ever been charged/accused" I checked yes, then wrote an addendum simply stating the date of the citation, the charged offense and the fact that all charges were dismissed. My approach is to avoid frivolous details/explanations/excuses and just give them the honest information. They are likely only asking these questions to ensure that you could pass the bar, and I don't see the bar turning anyone down for an underage drinking citation.
Of course, I haven't seen any acceptance letters yet, so take my advice with several grains of salt ;-)