1)Beat the Elite 4 ( 5 with the Champion ).
2)Complete the Sinnoh Pokédex.
3)Go to Prof. Oak and he will give it to you.
4)Enjoy your National Pokédex.
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I believe you meet him after completing the Sinnoh Pokedex. Then he will update your pokedex to national.
If you have the National Pokedex, Prof. Oak will be in a house in southern Eterna City.
First, get the National Pokedex. Then,talk to professor oak in eterna city.(he is at the bottom left corner of town as far as i know lol)
Professor Oak will not appear in the Lab in Sandgem Town until you complete the Sinnoh Pokedex. He will reward you the National Dex when you arrive there.
They roam the Sinnoh region after you talk to Professor Oak in Eterna City after getting the National Pokedex.