To finish the sinnoh dex (Pokemon platinum's,diamond, and pearl dex) you just need to see every Pokemon not catch every Pokemon.
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You find it in the solaceon ruins after you catch all the unown.
you cant. You can only do that on platinum):
you have to get the national dex when you see all 150 Pokemon orif you play platinum you will have to see all 210 Pokemon
Either find it or gain it "Gotta catch 'em all"
You can find articuno in the poketch marking app when he's all around