Instructions
Go to Goldenrod City in the center of the Johto region and enter the Goldenrod Underground through one of the entrances on the right side of town.
Speak to the man in the north of the underground tunnel to get a Team Rocket outfit. Use this to sneak inside the Goldenrod Radio Tower.
Climb to the fourth floor of the Radio Tower and you will find Rocket Executive Petrel in the Director's Office. He has five Level 30 Koffing Pokemon and a Level 32 Weezing, all of which are weak against Psychic-type attacks. Defeat him to get the Underground Key.
Head back into the Goldenrod Underground and use the key to open the door at the far south side of the tunnel.
Complete the puzzle in the first room by pressing the three buttons from left to right -- blue, green then red. This will open the gates and let you through to the next area.
Proceed through to the next room, battling past the Team Rocket Grunts which attack you, to find the Radio Director. He will give you the Card Key, which opens every door in the Radio Tower. This completes the Underground base, and you can now move on to chase Team Rocket out of the Radio Tower.