Enabling MultiSite in my WP Domains

Will someone please advise me how to activate my WordPress Multi Site features for my primary domain, and my sub-domains in the HostGator control panel?

A detailed description would be most helpful--in terms of line(s) of code, and where to insert them to activate Multi Site for the domains. Hostgator tech support told me that it's already installed in WP v.3.5, but that I need to modify my wp-config files to activate it.

Thanks for your time and expertise--I appreciate the help.

Best regards.