you start from the left, then the middle then the right
its.......imblack
You can trade Pokemon in the union room, the basement, and even in Goldenrodcity at the GTS place.
Try looking in the basement in burned tower in ecruteak city
3,2,1
beat exacutive patral in the radio tower and he gives you the key.
If you are talking about the one in Celadon, face the poster in the Game Corner and hit A.
The basement key is used in the Goldenrod underground. You get it while rescuing the Radio Tower from Team Rocket - specifically, after defeating a Rocket Executive disguised as the director.
you battle him
There isn't a basement. There is only the first floor.
There is no basement of the Old Chateau.
There are various little buildings with lit doorways that lead down to it. I believe one path to one is above the Pokemon Center and to the left.
you use it to open the door to the basement!