well Pokemon 151 in the national pokedex is mew so im just going to assume you just find a book on it like manaphy, and Pokemon 151 in the sinnoh dex is manaphy. to see manaphy, theres a book on manphy in mr backlots mansion. its on the table behind mr backlot when you walk into te room with him in it.
It's Manaphy and you can get its Pokedex data from a book at the Pokemon mansion below Hearthome.
In Pokemon Platinum Version, there are two Pokedex'es, the Sinnoh Dex, and the National Dex. In the Sinnoh Dex, the 151 Pokemon is Manaphy. In the National Dex, the 151 Pokemon is Mew.
Manaphy is 151
Pokemon number 151 is Manaphy
Platinum has more Pokemon 247 than DP and DP has the same amount of Pokemon 151
Mew. Trade or AR or hack.
manaphy
151 manapy 152 rotom
Manaphy
its manaphy151 is mew
manaphy i believe
Number 151 is manaphy