How do you get the elder of celestic town to show you Palkia on Pokemon diamond?

After beating Cyrus for the last time, go to celestic town. Go in the biggest house(in the top middle of town)and talk to the old lady. She should talk for a bit about being concerned about Mt. Coronet, and then show you a picture of Palkia.

After that you will have palkia on your pokedex.