In Pokemon Yellow, the lift key is in the Rocket Hideout under the Celadon Game Corner. One of the Team Rocket Grunts will mention the Lift Key before you fight him. Once you defeat him, he will drop the Lift Key. He should be in the northwest room on the 4th floor down (B4F). To get to the Rocket hideout, you need to use the switch behind a poster.
use the lift key to use the lift
It allows you to use the elevator in the Rocket Hideout, which allows you to reach Giovanni at the end of the hideout.
You can find the Lift Key in the Team Rocket Hideout on Pokemon FireRed. It is dropped by a Grunt on basement floor 4 after defeating him.
You don't
In Pokemon Yellow, the lift key is in the Rocket Hideout under the Celadon Game Corner. One of the Team Rocket Grunts will mention the Lift Key before you fight him. Once you defeat him, he will drop the Lift Key. He should be in the northwest room on the 4th floor down (B4F). To get to the Rocket hideout, you need to use the switch behind a poster.
use the lift key to use the lift
It allows you to use the elevator in the Rocket Hideout, which allows you to reach Giovanni at the end of the hideout.
You can find the Lift Key in the Team Rocket Hideout on Pokemon FireRed. It is dropped by a Grunt on basement floor 4 after defeating him.
in the bottom of the Pokemon mansion
You don't
To get the lift key, talk to one of the Rocket trainers you have already beaten. He will drop the key, then you can pick it up.
it is in the basement in the Pokemon mantion.
if you got it from the Pokemon mansion it opens the cinabar Pokemon gym
You have to battle all those Team Rocket grunts first. The Last One will drop the lift key.
You use it on the lift (1st Floor) to get access to Giovanni. This is your first battle with Giovanni.
You get the Lift Key on B4F when you battle a Team Rocket Grunt in the far north corner. He even says "Who has the Lift Key" before the battle begins. He has a Level 21 Zubat and Koffing. Once you defeat him, then he will drop the key.