Installing SSL to your subdomain works exactly as it works with a root domain.
Once there, find the domain which hosts the subdomain. Then click on "Actions" > "Subdomains" and there you can see your domain's subdomains. Click on the "SSL Certificate" button to view your subdomain's SSL certificate information. If you don't have an SSL certificate installed click on the link to open the installer.
SSL installation runs in 15 minute intervals.
For more information about installing SSL certificates, please see the how to install SSL certificate article.