Log on to your account, select 'Manage Domains' and select the domain you wish to alter. It needs to be using their nameservers ('Transfer DNS Back to Us' if required) and you will have a 'Host Management' section. Choose 'All Host Records' and you have the 'cname' option there. It's also on 'URL Forwarding' too if you use that. I don't know about putting files anywhere, I guess you'll have to host it on your servers to do that.