the key you got from the team galatic grunt dropped is not the one that's for the team galatic ware house you need to open the ware house door with that key and fight your way through it to find another key and that one is for the team galatic main building. You can get additional info here at the related link. (good luck!)
It,s in eterna city
you will get the storage key from the galactic grunt in front of the building and you will use theat to open the door in the storage room, after that you go through the building until you reach a dead end, at the dead end there shold be a door you need the card key to open. there is a poke ball beside it, pick up the poke ball, TA DAH card key
In Pokémon Pearl, after beating Candice, head to Lake Acuity. You'll see your Rival and a Team Galactic Admin Jupiter battling. Jupiter leaves. Head to Veilstone City. Head over to the big black building in the back of the city, this is Team Galactic's Headquarters. Talk to the Team Galactic Grunt that's standing outside and he'll drop the Storage Key. Pick it up and head into the Warehouse area which is the area in which you picked up the Fly HM earlier in the game. Use the Storage Key to open up the door that is inside the Warehouse and you'll be allowed through. Go through the building and you'll eventually find the Galactic Key. Leave the Warehouse building and enter into the main doors of the Galactic Headquarters. Open the locked door on the first dor and proceed up the stairs. Continue through the building and you'll eventually do battle with Cyrus, the Boss of Team Galactic.
Right outside the galactic hq, talk to the grunt and then looker will come and give you the storage key. The building in front of you is not the warhouse though, you have to go around to the upper left hand corner of the town and there will be the warehouse where the "Looker" will be there to unlock the door inside for you. i had this same problem and i was trying to use the key for the wrong door the whole time!
next door to the Pokemon Centre in the Survival Area. it is unlocked after you have completed the Stark Mountain mission where you finally defeat Team Galactic.
after you get galactic key go to the HQ with sattilite dish go to the door
It,s in eterna city
go to galactic headquarters in veilstone city then you need a galactic key and the storage key to open the door of team galactic's door
you will get the storage key from the galactic grunt in front of the building and you will use theat to open the door in the storage room, after that you go through the building until you reach a dead end, at the dead end there shold be a door you need the card key to open. there is a poke ball beside it, pick up the poke ball, TA DAH card key
All you have to do is get the key from the storage building beside it. the key to that door is given to you by a guy outside of the building that got fired.
All you do is go in front of the team galactic veilstone hideout and talk to the guy in front. then he will run away and drop a key. so that's how you get in.
go to the warehouse it opens that first door
The storage key is used to open the locked door in the small warehouse in Veilstone city where you can get the HM02 Fly (that's in the warehouse after getting the professor's assistant's pokedex back, you don't need the key for it). After unlocking the door you go down the stairs and battle a bunch of galactic grunts, and later you can find the galactic key and open the locked door in the galactic main building (it's the building that the first grunt dropped the storage key in front of earlier). After going down the stairs there and battling more grunts, if you defeat Saturn you can save the lake guardians (that's Uxie, Azelf, and Mesprit). Hope this helps!!
It opens the door to Cyrus the main boss of team galactic. Be prepared because after him you must defeat either Mars or Jupiter, they are very strong at that stage.
Talk to the Galactic Grunt by the satilite and he will drop something. Pick it up and it opens every door.
It is to the left of the Team Galactic Headquarters (Galactic Veilstone building), but you have to go around the Pokemon Center and Veilstone department store. Once your behind the department store go up the stairs and there is the door. (Galactic grunts might be guarding the entrance, so beat Maylene, the gym leader. To get to her gym walk to the very bottom of Veilstone City, then walk al the way to the right. Go up the stairs and continue walking up until you get to her gym. It is located here for both Diamond and Pearl.
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.