GREYED OUT: Create Ringtone option in iTunes store menu

Trying to create ringtone. I'm in the store, find a song with the ringtone bell icon, click on the bell - nothing happenes, go to menu bar under store and see "create ringtone" but it's greyed out! Help! I bought the song already thinking I could do it through itunes once I owened it, but you can't do it that way. You have to create it, then buy it. But with it being greyed out no matter what song, I can't do it...

Thanks,

Kim