Draco Plate: hidden behind the Dialga/Palkia statue in Eterna City.
Dread Plate: in the Old Cheateau, at the top of the stairs, go through the door to the north.
Earth Plate: in tunnel to the left of Oreburgh City, go north up the path, then follow around and take the stairs to the basement, use Surf to the far left of the cave and then Strength.
Fist Plate: Route 215.
Flame Plate: in Stark Mountain, hidden in the wall at the end of a path, on the left side of the big room.
Icicle Plate: Route 217.
Insect Plate: hidden in the wall to the right of the Old Chateau.
Iron Plate: hidden in the far southeastern corner of the third floor on Iron Island.
Mind Plate: on the seventh (last) floor of the Solaceon Ruins.
Sky Plate: Victory Road exit.
Spalsh Plate: Route 220.
Spooky Plate: Amity Square.
Stone Plate: Mt. Coronet in Diamond/Pearl, found in the Underground in Platinum.
Toxic Plate: Great Marsh (hidden in the northeast corner).
Zap Plate: hidden in front of the Lighthouse in Sunyshore City.
No, you do not need all of the plates to catch Arceus. You can use the plates to change Arceus' type, though. Arceus is only available by event (which has long since passed). You can also use an AR (action replay) to hack the game and get an Arceus, all without plates.
There is no Unknown Plate for Arceus, there are only plates for each type.
You get them through out the game
no you must have plates ex: thunder plate or sky plate
On the S.S.A Aqua.
See the following link for a complete listing of Plates and their locations: http://bulbapedia.bulbagarden.net/wiki/Plates
arceus because if you have plates you can change its type
Arceus is a Normal-type Pokémon but if you give it plates to hold it will change its type to the plate's type.
Arceus is a normal type. however if you give it one of the special plates you find it will turn into the type the plate is.
That'd be Arceus.
If you have Arceus, then all you have to do is give it one of the plates that can be collected around Sinnoh, for example, the Splash Plate. Then the Arceus will change its type.
if you give them to an arceus the arceus would change types, for example a flame plate would obviously make him a fire type! Plates also power up moves of their type.