If needed, turn off Yahoo's spam protection for a few minutes, and see if you can have the activation email resent for the name that you did want.
If that doesn't work, visit the Chatroom when you are online, and ask to speak to an admin. If they are not around, leave a message with one of the mods/supermods. Let them know what happened, and be sure to be relatively nice/pleasant to talk too.
This isn't a guarantee, but it is your best bet.