Its best to feed it 3 pokeblocks made from Pamtre Berries. If you dont know where to get one, go to the berry masters house and talk to his wife, tell her "Challenge Contest". It'll be the only Pamtre berry in the game from what I remember, so dont make it into a Pokeblock until you've grown enough of them. I suggest at least 4 so the you can blend 3 pokeblocks and have a spare berry left over just incase you want another Milotic for whatever reason.