You Go to the Jubilife tv station in Jubilife city. Go up to the third floor, and this dude will ask you questions. answer them, and he will give you a mystery gift
arceus (if you don't want to use a cheat to get it, go to pokemon.com and in maybe a few days or weeks it'll be on mysterygift in platinum)
To unlock Mystery Gift, you have to talk to a man on the 2nd floor of Jubilife TV. Tell him 'everyone happy' as the first answer and next is 'wifi connection'.
its a secret place
It evolves via happiness
only if you transfer it from emerald via pal park
you get the special key through the mysterygift
You cannot find a Grotle in Pokemon Platinum. You must start out with a Turtwig and then evolve it or get a Grotle via trading.
At an event (toys r us) or with Pokemon Battle Revolution.
you could transver tro via mission
yes you can honest truth
Yes via TM.
Yes. It doesn't make a difference where the Pokémon comes from whether it's from Diamond, Pearl, via migration, via the Global Terminal (or Global Trade Station) or via Friend Code Wi-Fi.