Unable to change my nameservers

I’m trying to change my name servers to redirect to Hostgator. I have loged into my wp.com account where I have purchased my domain name athomewithrebecka.com. I’m unable to get the option for changing DNS from my dashboard as shown in the tutorial. The only option I get is to “add domain to blog”. After trying this option I get this message…”This domain has already been mapped to a WordPress.com blog.”
How do I change the nameservers so I can register my site with Hostgator.