Adsense for domain is only for the domains you have registered or owned but haven't hosted yet.Don't messup your working domain.
here is one of easiest way to set up adsense for domains. This method works with all kinds of domain names you use for your sites .But here I have used co.cc domain for example as this method works best on this domain.
If you want co.cc domain register free domains here-jus click here
here are steps to setup adsense for domains
1. First log in to your adsense account then click gat ads from adsense set up tab.Click Adsense for domains
2. type a domain name without www. example if you own www.guitarmantra.co.cc type only guitarmantra.co.cc then click add domains button