Starting on the 28th of September a new Mysterious Gift appeared for Platinum players. Oak’s Letter triggers the ability to find and capture Shaymin on Route 224.
The procedure is fairly simple. First, if you haven’t done it already for the Secret Key, you must unlock the Mystery Gift option by following the steps on the included leaflet in your game case, and talk to the Jubilife TV Producer. When you select that option from the title menu you will receive Oak’s Letter over your WiFi connection. Once picked up from the postman at the nearest Pokémart you will want to head to the exit of Victory Road. Make sure to have the Surf, Waterfall, Rock Climb, Strength and Defog HM moves in your party, then find the path to the right of the last area of Victory Road which should have been opened after defeating the Elite Four.
Move through the caves, then to the edge of Route 224 to find Oak in need of your help. You’ll find yourself face to face with a level 30 Shaymin ready to be caught. What about the Sky Forme though? Well make your way to Floaroma Town with your new Shaymin in your party and a woman on the main path will hand over the Gracidea Flower. Use this key item to transform to Sky Forme, though it will change back at night, if frozen, deposited in a PC or traded.
Note that there is a lot of time, but this Mysterious Gift will no longer be available after November 8th. Read all the details on the page set up for Shaymin at the official Pokémon Platinum Version website.
[Edit: European players also have access to this gift, and likely other regions.]