#'s 151 and 152 in pokedex

Capture or Cheatcode

If you are talking about the Sinnoh Pokedex;
151 is Manaphy, who I have seen but have not caught. I must have met a trainer in the game who had one.
152 is Rotom, who is hiding in the television in the Old Chateau. Go there at night, go right up to the TV and press the A button. Rotom will appear. Choose a team that are strong against electric and ghost moves.
If you are talking about the National Pokedex, 151 is Mew,152 is Chikorita. I haven't seen either of them in this game yet. I hope this helps.

Sinnoh-
No. 151- Manaphy- Look in a book in the Pokemon Mansion on Mr. Backlot's desk in his office. The picture of Manaphy is in there. Also, Manaphy is not catchable during the game.

No. 152- Rotom- Go to the Old Chateau at past 8:30pm and go to the room with the TV up the stairs and through the top door. It is the second to the left. Examine the TV. It says it has a malevolent feeling and it is as if it is staring. Then you have a choice: Would you like to thump the TV? Say yes and it will say in the TV is a Pokemon. Then Rotom will appear.

National-
No. 151- Mew- You must migrate this Pokemon from Emerald to obtain in Platinum, also the only way you'll ever see this Pokemon in Platinum. I do not know how to obtain this Pokemon in any game.

No. 152- Chikorita- You have to migrate this Pokemon from Emerald to obtain in Platinum, also the only way you'll ever see this Pokemon in Platinum. I believe this is a choice of three Pokemon in the beginning.