beat exacutive patral in the radio tower and he gives you the key.
You can trade Pokemon in the union room, the basement, and even in Goldenrodcity at the GTS place.
If you are talking about the one in Celadon, face the poster in the Game Corner and hit A.
you use it to open the door to the basement!
sorry you have to trade it to Pokemon soul silver
you find the basement key in the underground path
Soul silver is AWESOMENESS!! yes GET IT
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 should iook it up on youtube and do it from there
According to Serebii.net there are 493 Pokemon in Soul Silver.
You can't do Pokemon contests in Pokemon soul silver.
Why Soul Silver is about the original Pokemon Silver game for game boy. Soul Silver has some more bonuses.
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.