Happiny is the only Pokemon that can evolve from a Oval Stone in Pokemon Diamond, Pearl, and Platinum. You will need to max its Happiness during the morning or day, let it hold an Oval Stone, and then level it up once. It will, then, evolve into a Chansey.
You have to trade for Happiny holding an oval stone or go catch Chansey on route 209 and if you do one of those Happiny can evolve into Chansey who can evolve into Blissey or you can catch Chansey like I did and evolve it into Blissey.
You just evolve it by making it hold a stone (an oval one..because of the egg thing you know..) and raising it one level during the day To evolve Chansey into Blissey, you max out Chansey's friendship and level it up.