How do you catch manaphy in Pokemon soulsilver?

Answer 1

You can't, as far as I know. You have to trade it from your Pearl, Diamond, or Platinum game, after getting it from your Pokemon Ranger game. Or get it from a friend.
There isn't a Manaphy encounterment in Soul Silver (unless you cheat), but there is a Manaphy egg event for Pokemon Ranger Guardian Signs and you can trade the egg to soul silver from Pokemon Ranger. You can then hatch the egg and it will be a Manaphy and the Manaphy is special because you can breed it to get a Phione. Also there might be a Manaphy event for soul silver coming up sometime later this year (2010). By the way you need to have a Wifi connection set up and you need to connect to wifi to get the Manaphy egg and the Manaphy from the event that may happen.
only if it's traded from a different Pokemon game.