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.
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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...