Here is a quick rundown of how to unlock the new forms of Giratina, Shaymin, and Rotom in
Pokemon Platinum

Giratina(Origin Form)- You need to go to the Turnback Cave(off Route 214) in order to
transform Giratina to its other form outside of the "Torn World". Follow the Turnback Cave
as you would in D/P in order to get Giratina. But instead of Giratina, there will be a
portal. Enter the portal to get the Platinum Orb. Give this to Giratina to hold to
transform it into its Origin Form outside of the "Torn World". Giratina's new ability will
be Levitate giving it full immunity to ground type moves. Also its offensive and defensive
stats will be reversed in its Origin Form. It will also boost its Dragon and Ghost attacks
by 20%. If this item is ever taken off, Giratina will turn back into its normal form until
you reequip the item.

Shaymin(Sky Form)- This can only be obtained with the EVENT Shaymin that comes in a
Cherish Ball. If you don't have this specific Shaymin you can't get its held item,
Glacidea Flower. However if you do have this Shaymin, go to Floaroma Town and talk to the
blonde girl on the dirt path. She will give you the flower to give to Shaymin. It's Sky
Form has an added flying type and can now lean Air Slash. Its special attack and speed
stats will greatly increase, however its defensive stats will lower. As long as Shaymin
continues to hold this item, it will be in its Sky Form in battle.

Rotom(Possessed Appliances)- Once again you need an event to access the forms. This time
you need the Secret Key item. Once you have this item, go to the Galactic Building in
Eterna City. Find a spot between the bookcases and use it on the wall. A secret area will
open where there will be several appliances. This works the same way as Deoxys in
Veilstone City. Just go up to one of the appliances select it. Rotom will change form. In
these forms its overall stats will increase. However its HP will remain the same and its
speed will lower slightly. Each form also has a special move Rotom can learn. These moves
include Blizzard, Hydro Pump, Overheat, Leaf Storm, and Air Slash. Rotom will always keep
its Levitate ability and stay in its Ghost/Electric type regardless of its form. If you
want to change back into the standard Rotom, just reselect the appliance you chose and
Rotom will change back to normal.

*Note that you can also use cheats to access the items, although cheating isn't the most
honorable way to do things. Also just to restate, Platinum Orb does not need an event, but
Glacidea Flower and Secret Key do.