Click on the 'm' buton next to the domain name you wish to modify. This will take you to the Domain Name Control Panel, a page that allows you to edit all settings for this particular domain.
The Domain Locking option is in the box on the center of the page. Checking the box enables the locking while unchecking removes the lock.
Please allow approximately 24 hours for the status to update through the global registry. You can check domain status at http://registrar.verisign-grs.com/whois/ and conducting a whois search for your domain.