Map Subdomain?

Have a WP Multisite in development, with three different sites. We have multiple domains mapped to each of the sites and it’s working perfectly. But now we’re trying to add a subdomain that points to one of the sites, and it’s not working–you just see a blank page with links to an “OLD” directory and a wp-config file. (I’m not even sure where these files might be on the server–they don’t appear to be in any of the main directories.) Is there something special you have to do to get subdomains to work?

The page I need help with: [log in to see the link]