Oued Souaffine 41,
If you come by taxi, ask for Batha fountain. From there you can take a man with a luggage cart to take you to Oued Souaffine. This will cost you a couple of euro's.
If you don't like this option, it might be better to have a map of Fez Medina (this is a very important help to get around) and get to Oued Souaffine.