Ok first of all Team Plasma isn't in Pokemon Diamond, Pearl, or Platinum. Second of all what idiot told you Team Plasma is in Pokemon Diamond?
the team galactic boss gives it to you at the team galactic hideout!
there is no underwater galactic hideout, the closest hideout you would get to underwater is the aqua/magma base in ruby and sapphire
Team Aqua's hideout
a master ball
You can find team aqua's hideout in the east side of Lilycove City in Pokémon Sapphire.
Team Magma`s hideout is where you go down from the cable car east of lavaridge. Team Aqua`s hideout is on the East side of lilycove, you surf northeast to get there.
it is at mt chimney
Team Magma Hideout
You get it from the team Aqua hideout.
In team aqua's hideout
You find it in the team aqua hideout.
you find it in the team aqua hideout