Where to change site URL per Network Setup recommendation?

Hi. First, I read the other thread on this subject but really couldn't understand what the instructions were. Shortlink: http://bit.ly/1i93T1A

I've set up only one multisite before, and it was a fresh install of WordPress. This time I'm trying to create a multisite with a domain that already has a live site. Maybe that's why I'm seeing this message during setup: "We recommend you change your site URL to welcometojoseph.com before enabling network."

See attached image.

I tried to change the Site Address and couldn't, and I worry what will happen if I try to change the WordPress Address. Or is this a dumb question and I'm supposed to change it in the line directly below that says "The internet address of your network will be"

I'm on Hostgator and have already setup the wildcard and it's showing up in the Subdomains section as *.welcometojoseph.com

Any instructions to help me move forward will be appreciated

(BTW, I hope this doesn't double-post because I encountered a server error the first time I tried to post it.)