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.
Chat with our AI personalities
you press the a button on it when you get the odd keystone. to get spiritomb there,you have to meet 32 people underground.
You walk up to the crumbled tower and press "A" and it will say the odd keystone looks like it will fit.
It is impossible to not be able to put the oddkeystone in unless you have a glictch or you click no when putting in the odd keystone
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. =]
You can sell it to the ore maniac in Icirrus City for 3000 Pokedollars.