It's hidden in Celestic Town.
from the foreign guy
When playing the Pokeman light platinum destroy huge rocks you have to use route 509.
celestic town, wild poliwhirl
you must HM rock smash.
you need rock climb
it for the powerful Pokemon home "spritomb!"
You can use any Pokemon that is able to learn Strength to push rocks.
What?!?
U can sometimes find them on wild poliwhirl
The platinum nut
Use the hm strength.
You need the move rock smash