Galctic Headquarters is actually the Team Galactic Veilstone Building, but you'll need a Galactic Key to enter most of the doors.
Chat with our AI personalities
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.
Check out this site:http://www.anime-stadium.com/index.php/Ongoing-animes/Pokemon-Diamond-and-Pearl-Galactic-Battle-55MB-HQ-MKV
give porygon (or possibly porygon 2) dubious disc ( i think you nick it from galactic hq) and trade him.
In the galactic warehouse to the left of the Galactic HQ
You just have to beat all of the galactic grunts and you will be at the top.