To add any DNS record type, begin by activating "Free DNS Management" for the domain.
Upon activation, you have access to add an "A" record. Simply specify your host name, like "@" or "www", select "A (Address)" from the "Record Type" drop down and then enter the IP address for it's destination. Depending on your domain's registrar, you can add as many sub domain A records as you like.
In the example below, A records for http://www.yourdomain.com, http://yourdomain.com, and any sub-domain http://*.yourdomain.com will be pointed to the server at the IP address 64.50.91.234.