you can't looker is only in Platinum
You talk to the galactic grunt and he will run away. Then go to the small building to the left and the looker will be there. Follow the looker and getthe galactic key. Now you can get in the door in the galactic building in veilstone.
the team galactic boss gives it to you at the team galactic hideout!
you have to beat the galactic warehouse, and when/if you beat it you then go into the galactic HQ by entering from the galactic warehouse, and then Looker will come open the door.
You may not join Team Galactic in Pokemon Pearl or Diamond.
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.
Team Galactic boss is in M.T Corenet near snowpoint city.
veilstone team galactic base veilstone team galactic base
Both Pokemon Diamond and Pearl have Team Galactic as the main antagonist team.
go to the team galactic base and beat them then he will give it bak and looker will gome
The Team Galatic warehouse key can be obtained (after you've seen the confrontation at Lake Acuity) from the Galatic Member outside of the Veilstone base, just talk to him and he drops it. Looker will take it to the warehouse entrance.
at the stone pilar
in diamond and pearl it's in the wherehouse next to the gelactic buldind in a town but in platinum you defeat the grunts and give dawn her pokedex then looker will come and fallow him the he will give you the key. p.s. do not erase my answer