* Shaymin Sky Form is introduced in Pokémon Diamond and Pearl. SoulSilver came out before these games.
* Shaymin Sky Form is a special form. It's triggered by using the Gracidea Flower, which is not available in SoulSilver.
What you CAN do in SoulSilver:
* Catch Shaymin in its Land Form: Shaymin Land Form is available in SoulSilver via an event that took place a long time ago. It's unlikely to be available again.
* Trade for Shaymin Land Form: If you find someone who still has Shaymin Land Form from an event, you can trade for it.
If you want Shaymin Sky Form:
* You'll need to play Pokémon Diamond, Pearl, or Platinum. These games are the only ones that introduce Shaymin Sky Form and the Gracidea Flower.
Let me know if you have any other questions about Pokémon!