ok so once you finish the shinoh dex in pokemon pearl or diamond, you get the national dex. But then what do you do? where do you go to see all the national dex pokemon?
in the island where the sailer in snowpoint city takes you and if you beat the pokemon leage and cynthia go to canalave city and go into the salers home and talk to the boy and he'll say something about darkri and go to the sailer outside of it and he'll take you to full moon island and you see cresselia plus giratina is in spring path in tunrback cave
You can find the National Dex Pokémon in the Fight Area, Survival Area, Stark Mountain, Trophy Garden, Swarms (Go to the Sangem Town and talk to the girl in the house next to the beach, she will tell you where you can find Absol, Surskit and other details). Then you will need to use the Poke Radar to find the rare Pokémon.
You can't see all the pokemon in pearl...unfortunately. However, you need all the other gameboy pokemon games (pokemon fire red) and you migrate them into your game. You can do this by going to sandgem town, going as far south as you can, then going east. You should hit water, surf across it, and you will come to some land. Keep on going east, and you will find a building. In there, there are options where it will tell you to put your old gameboy games in, and basically bring the pokemon you have in that gameboy game back to your diamond/pearl game. Yes it's hard, but it's worth the fun.