Pokemon diamond: my pokedex wont go past 149?

i have diamond, and i have beaten the elite 4 and everything and im trying to get the national pokedex and the poke radar. but whenever i look through my pokedex it stops at 149 at the bottow(dialga). but i still need to see palkia and manaphy to see them all but i have talked to the lady in celestic town and she showed me the picture of palkia and it will not register in my pokedex