As announced yesterday by the official Pokémon website, “Oak’s Letter”, the item giving you access to the Flower Paradise (and thus, Shaymin), will be available for download from September 28th to November 8th. Once you obtain the item, go to the north end of Route 224, where Prof. Oak will be waiting. After that, you’ll be able to go to the Flower Paradise where you can encounter Shaymin at Lv.30. If you have an american Pokémon Platinum version, be sure not to miss it!
Also, note that the Shaymin caught in Flower Paradise can be changed to its Sky Forme by using the Gracidea Flower on it.