You cant
Answer:Have a Pokemon that knows "cut" use the move from the menu while standing in front of the door.
upstairs in the Pokemon center first door
You stand in front the valley windworks door and press the A button say yes and you'll unlock it. :3 -.-'
you find him next to the door in the thropy garden
use the door its south of Solacaon.
get dialga palkia and giratina put them first in your party go in there and go in front of the painting and press a and a door opens up
walk out of the door!
in the look door in canalave city
Top of the Mountain. Through the back door...
you can find him some places if so he will be at the door
you insert the gba game in the bottom of the Nintendo ds and diamond on top then you go to diamond and then press start where dialga appears and then don't press anything. you will see something down and it says migrate from leaf green to diamond but first you need to beat the elite four and have the natoinal dex then you go to sandgem town go down then surf to that one place and then you talk to pro.oak then you go to that one guy that is standing in front of the door then you talk to him then he says...................................................... so then he'll let you get in there and that's where you get the Pokemon you migrated from leaf or fire to diamond or pearlso yeah.
Just beat the Cerulean gym -> Go to Bill's house -> Then the house that a police was standing in front the door would be unlocked and you can go through it to whatever route!