Legal Question in Criminal Law in Massachusetts

From what you're saying (i.e., that you had the charge dismissed before a magistrate), it doesn't sound like there is any charge to have sealed. If the magistrate had found, instead, that there was probable cause to charge you with a crime, you would have then been arraigned, and there would have been a charge entered on your record at that time. As it stands now, however, the magistrate ruled in your favor, and there is no charge at all on your record, so there is nothing to seal.