Where do u see diagalia in pokemon pearl?

Question

i only need to see him to complete my pokedex

Answer 1

Go talk to Cynthia's grandma and she "shows" it to you.

Answer 2

I cant remeber exactly but go to sandgem town and go into one of the houses talk to everyone and someone should show you a picture and that will complete you pokedex.

Answer 3

In order to see Dialga in your Pokemon Pearl version, first, you must Fly to Celestic Town.  After you fly to Celestic Town, visit the Town Elder's house and talk to both the Town Elder and check out the book that is lying on the table nearby where she is standing to get the 150th entry for your Sinnoh Pokedex :)

Hope this helps :)