If you need to activate wildcard subdomains for a domain name of yours then you can just add a subdomain, titled "*" (without the quotes) via the CPanel for the account in question. Access the Subdomains page and add the "*" subdomain with a DocumentRoot of /public_html (or wherever you want the wildcard subdomains to point to). The appropriate VirtualHost and DNS entries will be automatically added by CPanel. The Apache will also be restarted automatically.
This is useful when installing WordPress MultiUser (WPMU), for example.
If you need more information then please do not hesitate to contact us at firstname.lastname@example.org.