the galactic hideout but i think you need to beat the gym leader before you can get through.
you go to the old chateu.go find it .if you have a hard dude or dudet its in the top left corner next to a black and yellow and gray machine.
Get event on action replay.
you can but first you need the member card.
get the member pass from a Nintendo event and then go to canalave city to that locked door
you went to the wrong door . try the door near the sailor in canalve city.
You can get it from Bebe(next door to the Pokemon center in Eterna city), or you can check out the trophy garden, it will appear there occasionally.
There is go to the Galactic buliding WITH the secret key AND ROTOM.So goto Eterna city and the fancy bad building and youfindasecretdoor
It opensa secret door in the Eterna Galactic building where you can change rotoms forme.
It,s in eterna city
i don,t no but if your talking bout how to go underground in Pokemon platinum you have to go to eterna city and enter the house next door to the Pokemon center. hope i helped! :D
You need action replay or go to an event if you want to do this.If you open the door rest into the bed.You'll be on new moon island.enter a little doorway,then...you'll see Darkrai!!
If you're talking about the Poketch app, talk to the girl closet to the door in the Pokemon center of eterna city. If you just want to find out how happy your Pokemon are, talk to Dr. Footstep, somewhere on the beach east of pastoria city.