I don't believe it does anything in Soul Silver. I have one and I looked everywhere to use it,but I didn't find anything. The only place I know to use it is in Pokemon Diamond and Pokemon Pearl versions at the Hollowed Tower,which after you talk to I think 30 people underground you get to encounter Spiritomb. But if you do find what an Odd Keystone does in Soul Silver or Heart Gold,I'd be glad to know. =]
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You can't. The Odd Keystone is only obtainable in Diamond, Pearl and Platinum. You will need to trade a Pokemon with an Odd Keystone to get it on SoulSilver.
You find the odd keystone and go to the tower. Then face it and press a.
you press the a button on it when you get the odd keystone. to get spiritomb there,you have to meet 32 people underground.
in a house
You walk up to the crumbled tower and press "A" and it will say the odd keystone looks like it will fit.