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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?
Chuck Norris
On the southwest of Charpos Island
it's on five island at the bottom of path going downwards.
sapphire
she don't come back
you go to slateport city and go to the place where the boat is and you will see a group of people. go inside and you will see the boss. then go back to lilycove and go to team aqua's hideout and you can go inside.
east of lilycove
You go to island five then go inside the team rocket hideout the you battle some people then you battle Jesse and James and they Will give it to you.
after getting into team magmas hideout
There is no way to get inside after they close it unless you saved before you went in
because there is a team magmas hideout near lavaridg town first go there with the help of cable car in the you will find a cave go there search for there boss