After you get entered into the hall of fame go back to sendoff spring and go into the cave. Once you get to the room where it looks clear with no fog go to the center of the room an jump into the Distortion world. Keep following the path until you find an item. It is called the Gresious Orb let Giratina hold it and he will change
After you beat turn back in platinum you could go inside the turnback cave then go find three pliers then go into a portal then your in the distortion world then go find a ??? That is the item that makes it happen's hope it helps
You have to go back in the distortion world go where giratina was and you will find the grieseous orb where he used to be.