Don't even bother with the "I was young and dumb" sob story. Just answer the questions as asked. For example, from my GE interview:

CBP: I see you were arrested in July 2003.
Me: Yes, I was.
CBP: Were you convicted of anything?
Me: No, sir.
CBP: OK, I see that the four misdemeanors were nolle prossed at trial. What about the felony?
Me: It was withdrawn at the preliminary hearing.
CBP: Oh, I don't show that at all. Can you provide paperwork showing that the felony was withdrawn?
Me: I can contact the court.
CBP: OK, good. Just fax it to this number once you have it. You can't be approved until we get that paperwork.

I faxed over the court docket an hour later, and was approved less than 5 minutes after sending the document over. It turns out that CBP can see my local police records and the local county court files but could not see the district justice records where the prelim occurred.

Notice that the CBP interviewer didn't care one bit about the specifics or whether or not I was sorry, even though the major felony was the one charge he couldn't clear during the interview? This isn't a parole hearing so just be factual and honest: "I was arrested, I went through a diversion program, and the charges were withdrawn."

For what it's worth, the NEXUS CBSA background investigator called me directly to ask about those very same charges. She tried to get me to describe what happened in more detail - actually asking me "what happened when you were arrested?", but I stuck with "arrested for <charges>, all withdrawn, no convictions, where do I fax the records?" and that was that.