help with multisite

I seem to be completely failing to understand WP multisite. I know very little about hosting and file system management.

I did a multisite install when I first installed wordpress. It was installed in /public_html/wp. Currently http://www.theslowroad.org/wp/ is run from this install.

I also hold the domain melissakuhnell.com. There is currently a folder public_html/melissakuhnell.com which holds the old html and css files for this site. I want to use wordpress to build a new new site at melissakuhnell.com.

I tried to add it as a site in the Wrodpress Network Admin, but that failed.

How do I accomplish this?