To obtain Duskull or Dusclops in Pokemon Diamond, Pearl, and Platinum you need to go to Route 224 and use your PokeRadar and 50% for Dusclops and 25% for Duskull. To obtain the PokeRadar you must SEE all 150 Pokemon in the Sinnoh Region.
Here is a link for all 150 Pokemon in Sinnoh and their locales: http://pokemondb.net/pokedex/game/diamond-pearl
Just click on the Pokemon you want to find and then just go to the bottom of the page and you will see all the different ways to obtain them in all of the games.
Hope this helped! :)