I think that shaymin's forms are exclusive to platinum as they require a giveaway from a Nintendo event. If it wasnt built into platinum pearl/diamond then you wont be able to change its form. It would be like trying to trade across Pokemon to lower generation games which don't support that Pokemon. If the method for doing it in platinum doesnt work then you simply cant do it.
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You can't change Shaymin into Sky Form in Pokemon Diamond (that includes pearl). You can only change Shaymin into Sky Form in Pokemon Platinum (that includes Heart Gold and Soul Silver). You can get the Gracidea to change Shaymin into Sky Form.
Actually just wait until night without shaymin holding the flower thing
You can get Shaymin in sky form only in Pokemon Platinum Version.
If this is in Diamond or Pearl, it is impossible to receive the Gracedia flower or Shaymin Sky Form. In Platinum, you must have a Shaymin IN YOUR PARTY and then ask the girl. She should give you the Gracedia flower, but it does not work at night or on any other Pokemon.
by the way you spelt shaimin rong it shaymin. go to event to get it on diamond or pearl then trade it to platinum. to get sky form (on platinum) go to floroma. talk to woman with blonde hair. she giv u gracidea. use this on shamin during the day and he change form. shaymins only sky form during the day and turns normal at nite. use gracidea again on shaymin the following day he become sky form again!