How to add additional sites to your WordPress Multisite installation?

Hola, amigos!

I am really excited about using WPMS. I'm super-green when it comes to WP. So, please refer me to the correct post, if my question has already been addressed.

I have an addon domain (via hostgator & it's NOT my top-level domain), for which I've activated WPMU.

I want to add a different domain, but WPMU won't allow me. I go to: My Sites >> Network Admin >> Sites >> Add New
It only allows me to add a subdomain to the existing addon domain that has WPMU activated.

What do I do? I'll be happy to supply you will more info, as directed.

Saludos!