There is no Unknown Plate for Arceus, there are only plates for each type.
See the following link for a complete listing of Plates and their locations: http://bulbapedia.bulbagarden.net/wiki/Plates
Arceus is a Normal-type Pokémon but if you give it plates to hold it will change its type to the plate's type.
You have to give it the plates you find. He changes to the type that it would normally boost the power of.
Arceus doesn't have an evolved form; however, you can change his elemental type based on what plate you make him hold. The plates are scattered throughout Diamond/Pearl/Platinum. -DJAikou
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. however if you give it one of the special plates you find it will turn into the type the plate is.
Arceus is a Normal-type Pokémon but if you give it plates to hold it will change its type to the plate's type.
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.