Dropsy is usually a symptom of another condition-- it suggests organ failure in the fish, sad to say. That having been said, some people have had success if it is caught early. I think that if you are SURE it's dropsy, I would go directly to Maracyn, assuming it is safe for bettas. (that, I don't know.) Still, Lexi, if it doesn't work, know that you have done everything you can to help the poor fish.
Are you positive that it is dropsy? Is the fish pineconing?