there is another entrance on the left
defeat team galactic boss at the team galactic veilstone building.
First you beat the Pokemon league.after that go into the cave in front of the Pokemon league.u will find a man blocking the entrence to the other part of the cave.talk to him.he will let u in
you find it in the galactic HQ
You may not join Team Galactic in Pokemon Pearl or Diamond.
Both Pokemon Diamond and Pearl have Team Galactic as the main antagonist team.
You will need Galactic key and this how to get Galactic key you will need to talk to a Galactic team mate by the Galactic HQ.
With the Galactic key you can unlock any locked doors in the headquarters.
go in yellow circles and stairs
You can't join
All you have to do is Batlle him and get the Galactic key card
you use it in the warehouse i think
somewhere