Like the other agents said, depending on the nature of your complaint, first start with the office manager or owner. The other avenue is to file a complaint with the State Real Estate Board. Check out the states site very informative. Good luck.
I'm so sorry to hear you are having a bad experience and kudos to you for speaking out about it. First you need to confirm if they are indeed a realtor meaning a paying member of a realtor association. You can do that by searching for them at realtor.com. If they are member, I would start by calling the MA Association of Realtors on their customer service line at 800-725-6272. You can also file a complaint with the Division of Professional Licensure for the State of Massachusetts on their complaint line at 617-727-7406.
You would go to the office manager and see if they can explain or remedy the situation. If you have a complaint they will tell you what to do. Bad customer service doesn't in itself warrant a complaint, but whatever it is (money whatever) I hope you can get things resolved.