in about a week
(the above answer was written on 26 Sept 2009, so a week from that date is 3 October 2009. Although I cannot confirm the answer itself.)
Because it is not an event shaymin or is in diamond or pearl
Shaymin cannot be legitimately obtained on Pokémon SoulSilver. It must be traded from Diamond, Pearl, or Platinum. To get Shaymin in Diamond, Pearl, and Platinum, you must either use an Action Replay or take advantage of an event.
The only way to get shaymin is to do the event in pokemon Platinum/Pearl/Diamond and to transfer it to pokemon black/white
It will be in February, in London, they are going to offer for free a Shaymin.
Its much easier to trade for it, You need a special event to get it in your game
It is obtained from a Nintendo event in diamond, pearl and platinum or an AR (action replay).
In the 4th Generation (diamond, Pearl, Platinum) there is no trainer in the game that owns a Shaymin. A player can own a Shaymin if they activate the Oak's Letter event.
You can't catch Shaymin in Heart Gold or Soul Silver; it's an event Pokemon from Diamond, Pearl and Platinum. You have to transfer Shaymin from those games to have it.
There was no Shaymin event for Soul Silver, only Diamond, Pearl and Platinum, which ended on November 8th 2009
shaymin is an event Pokemon only and is not catchable unless you either hack the game or got to an event and unlock it there
you must send him over from a pearl or diamond or cheat (Action replay or hacking)
If by "shamin" you mean "Shaymin", then you can't. It must be traded from Pokémon Diamond, Pearl, or Platinum.