You need the Mauville badge, go to Mauville and fight the gym leader, but you must first fight Wally to get to the door open.
Head back to Viridian City and challenge the gym there (it will be open now) to get your 8th badge. After that come the Pokemon league.
To open the door, you need to play Blue, blue, green, yellow, yellow, red, blue, green.
You need to go to the bottom floor and take the stairs and ladders. Yes it means going against a few trainers and actually working but Its for a badge. (Hope it helps)
You can find the key to unlock the door in Pokemon mansion
Open the door and challenge the gym leader.
You need the Mauville badge, go to Mauville and fight the gym leader, but you must first fight Wally to get to the door open.
Head back to Viridian City and challenge the gym there (it will be open now) to get your 8th badge. After that come the Pokemon league.
To open the dotted hole you need to use the hm cut
In Pokemon platnom the door opens magically when you catch the newest Pokemon CROWLEY caught at an event and when you go tho the door it should be open.
when you get to the door play this on your reed pipe (flute) and it will open :blue-blue-green-yellow-yellow-red-blue-green
The door eats yellow, blue,red, then green
Get a Pokemon who knows cut and use it on the door. You can't just press "A" at the door though you have to open your Pokemon menu and do it from there.
card cloning
the door will be open after you solve the crisis between kyogre and groudon.
Just push the silver vw badge in middle of boot door in it opens then
To open the door, you need to play Blue, blue, green, yellow, yellow, red, blue, green.