You need to provide more context, but I can try to define what it means: "Then he appeared" probably means he walked into the room, or he came into sight nearby. "what a fuss about nothing" means that somebody is very upset and it's not a big deal. Somebody is worried but there's nothing to worry about. "this treat's on me" means "I am paying for this" usually the context is in a store or a place where people eat or buy drinks.