Either catch them all see some and catch the rest or use the action replay cheat.
battle everyone
see all 210 Pokemon
you see all the Pokemon in platinum and trade from your other games like emerald, blue, fire red etc and that is how you find all Pokemon in platinum.
you have to see all the Pokemon in sinnoh
go to psypokes.com :) and you can see all about Pokemon games.
You have to see all Pokemon Pokemon. But not the ones on nationaldex.
all of them in the regional pokedex
go and see all the Pokemon in the sinnoh dex
To finish the sinnoh dex (Pokemon platinum's,diamond, and pearl dex) you just need to see every Pokemon not catch every Pokemon.
You have to see all of the Pokemon in the original pokedex.Then,go to Rowan in Sadgem Town.
Please see related links.
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