I don't use action reply, but i think i can help. I suggest you download an application called pokesav. It has an option to complete both the national and sinnoh dex on my action replay it already had some cheats on it and that was on it.
look on supercheats.com for it
you need to find 210 Pokemon to complete the pokedex
First complete the sinnoh pokedex, 210 Pokemon
Just go to Pokemon platinum central and click on pokedex and it shows you where to battle all the Pokemon in sinnoh.
NOTHING
you have to beat the Pokemon champion and later on in the game you will complete it
i think it's L+R+SELECT
There is only one legitimate way to complete the Sinnoh Pokedex. To complete the Sinnoh Pokedex you must trade for the missing Pokemon with Pokemon Diamond, Pearl or Platinum.
you have to go to Professor Rowan.
that's what im trying to figure out ! im not sure if theres a code for it :(
complete the sinnoh pokedex and talk to professor rowan.
That pokemon is Azumarill in pokemon platinum in the national pokedex