The System tab > Domain Management > Domains portion of the HSPC control panel is where you add domains and hostnames to your VPS. This will create the DNS entries for the hostnames but not sites associated with the hostnames.
To add a subdomain click on a domain you have added. Click on the Hostnames tab then Click on the New Hostname button. Enter the subdomain portion of the hostname and select the IP address (if you have multiple IP addresses). Click on the OK button to save the hostname.
The Site tab is used to create the directory structure and Apache web server entries for a site. Click on the site tab then click on Add Site. Select the hostname you created in the Domain Management section of the control panel. Click on the Next button. Select an administrative user and any other options you wish. Click on the Next button then finish creating your site.
You can also find more information on how to use the HSPC control panel at Help tab > Documentation > HSPcomplete Subscriber's Guide. Also, you can find a context-sensitive "(?) Help" link near the top right hand corner of the window on most screens.