How do you catch Palkia or Dialga in Pokemon platinum?

Answer 1

Go to Mt. Coronet through Oreburgh City, as if you were traveling to the Spear Pillar. Once you get to the outside part, use rock climb and enter the cave on the right. Use surf and waterfall to get to a small cave. Enter it to find the Orbs that are needed to battle Palkia and Dialga. Go to Celestic town and inspect the ruins or pictures in the cave. Then in the same city talk to Cynthia's grandmother (shes in the house on top of the city-biggest 1). Go to the top of Mt. Coronet to where Team Galactic was previously encountered. Dialga will be there. Catch it, then leave to Mt. Cornet and return to encounter the other one. I recommend you stock up on all kinds of poke balls and save before you battle each.

You might need to beat or catch Heatran first.

Answer 2

Once you have the Adament and Lustrous Orbs, go to Celestic Town and Talk to the Elder. Go to Spear Pillar and talk to a Blue Light and Dialga will appear. Go back into Mount Cornet and come back and a Pink Portal will be there. Talk to it and Palkia will appear. They appear at level 70, so come prepared. Please note that you must have beat the Elite four and the Champion. Hope this Helps!