To get the Odd Keystone in Pokemon Diamond, you must go to Route 208, where there is also a Berry Master that gives you Berries, daily. You can also receive a Poketch App from the Little Girl in the house. Along with the two Trainers in that Route, William and Hannah, there is a man with a red-coloured headband, who looks like the Karate Trainers.
If you go talk to him, he will give you the Odd Keystone, which you can use to help you get the Pokemon Spiritomb if you restore the tower on Route 209.
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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 walk up to the crumbled tower and press "A" and it will say the odd keystone looks like it will fit.
you press the a button on it when you get the odd keystone. to get spiritomb there,you have to meet 32 people underground.
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. =]