By Trading.
Here is a list of all 150 Pokemon: http://www.serebii.net/diamondpearl/shinoudex.shtml The last Pokemon listed (#151) is not required to get the national pokedex.
you have to see all 150 Pokemon in the sinnoh poekdex
there are 150 kinds of Pokemon that you can see and/or obtain. Unless you have friend trade you Pokemon that you can only get in pearl to you, you'll never obtain them all. In Pokemon diamond and pearl all you have to do is see the Pokemon and you get its data unlike the older versions where you have to obtain them.
all sinnoh pokemon tips:just battle all wild pokemon.trainers,gymleaders,and elite4
Talk to the Proffessor; he'll give you a National Dex.
It's pretty easy, all you need to do is walk there.
easy-find someone with it and thn migrate it- find it in the pal park all day scyther is not in Pokemon pearl
Which type of Pokemon do you have but if you have diamond/pearl you can get the Pokemon modifier cheat code and put it in your action replay
Press (a) continuously {start} is automatic faint and yeah that's all ^_^
In diamond and pearl, you must have seen all of the 150 Pokemon (seen, not captured) in you're Sinnoh Dex.
See all 150 Pokémon and beat the elite 4, then talk to Proffesser Rowan.
all you have to do is see about 150 Pokemon is all and make sure they are all different (and try to catch them all because some are very rare and you can only see them once)