If you are on a server that currently supports IPv6, you can enable it by first logging into the customer portal.

Once you are in the customer portal, please click on 'Services' and find the hosting account you wish to enable IPv6 for.

Under that email account, there's a tab called 'Management Actions'.  If you click on the drop box, you'll see two links, Enable and Disable IPv6.

Please keep in mind IPv6 is still very much in beta, we do not recommend you use it in for a high-traffic website.