You can get the dawn stone from Mt. Coronet (lower level) between Oreburgh city and Hearthome city in platinum or by using the pickup ablity some pokemon have.
If you get it from Mt. Cornet then you have to enter at route 207 then turn right and head towards route 208 when you reach the rock you can smash, then turn right again then surf and then there is dawn stone right there!
(there maybe other ways to get it but this is the one that I know)
Make sure your snorunt is female or else the the stone won't work!