Previous answer: You can't unless you use action replay, hack, or trade.if you do action replay you people who use it are a freak and a cheat so shut your gob and stop using action replay ars holes
My answer: Most of the National Pokedex Pokemon can be found on the island with the battle frontier. The rest you have to either trade in from another game, or do a Wi-Fi event to get the other legendarys like Darkrai. I don't know if the Wi-Fi event Pokemon count in the NAtional dex.
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
you just have to play the game
fill your pokedex and battle more trainers
you have to win the champion and fill your pokedex. (^^)
Do you have Giratina? Do you have a national pokedex? did you get Heatrans? If you have a national pokedex you can go to Celestic Town and go in the cave and observe the inscription. Cynthia says that it may not be the three lake Pokemon. maybe it's Dialga, Palkia, and Giratina. I dunno. I'm kinda at a loss, too... Just try to fill up your pokedex, i guess...
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
you just have to play the game
Yes you can. You just have to fill up your pokedex and talk to pr.Rowan.
no they don't add Pokemon into your pokedex there
Fill up your Sinnoh pokedex. Here's something that will help you: Rotom.
To get the Poke Radar you gotta fill up the Sinnoh Pokedex, talk to the professor, and he will give you the National Pokedex and the Poke Radar.
use action replay
WAAAAAAAAAAAAAAAAAAAAAAAAAAH! get 1 at proffeser rowans lab in sangem town but u gotta fill up the sinnoh pokedex but i don't have 1
you have to see all of the Pokemon and then you get the national pokedex from Prof. Birch and Prof. Oak in Prof. Birch's lab after completeing the Sinnoh pokedex.
After the elite four, you fill up your sinnoh pokedex (by seeing all the Pokemon in sinnoh), and then go to Sandgem town, and talk to professior rowan in his lab, to get the national pokedex, and then explore the island that you take the boat from snowpoint city to get to, and fill up the national dex
you fill up the sino pokedex
fill your pokedex and battle more trainers