You have to go to the laundry pool after you have the letter from the girl at the Stock Pot Inn. Put the letter in a mailbox, wait for the mailman to pick it up, and follow him when he goes to deliver it. He will ring the bell, and a kid in a mask will come out. Follow the kid back inside, put on the Kafei mask, and talk to him. You have to talk to the mayor's wife to get it.