To purchase the add-on domain, just login to your main Sitelutions account at http://www.sitelutions.com/, then navigate to the Sitelutions Space Control Center and click the "Go!" link next to "Purchase Upgrades and Add-Ons." 

You will see several add-on options listed, and one of them should be something like "1 Add-On Domain and 1 Subdomain."  The cost for this should be $1.50/month, multiplied by the number of months remaining on your package.

After you purchase this, click the "[configure]" link next to "Add-On Domains" in the Sitelutions Space Control Center for the appropriate account.  You can then add the desired domain to your hosting account.

With our new InfiniSite and InfiniPro packages, however, unlimited add-on domains are included in the package. Simply head to your Sitelutions Space Control Center and click "manage" next to add-on domains.