Registration of a .es domain is unsupported by Dreamhost, but you can manage that domain here. What that means is, you can go to Manage -> Domains -> Add Domain and do everything related to hosting just the same as you could if you were hosting a .com. If you do that, Dreamhost will happily add your domain to its nameservers and take good care of making sure that everything gets pointed to exactly where it needs to go. The only additional step you need to do is to have your registrar (who isn’t Dreamhost) point to Dreamhost’s name servers.
If your registrar won’t let you change your nameservers for some reason, or if you don’t want to, then what I would second the suggestion of using a CNAME and invite you to post back with some details about what exactly you did that didn’t work, preferably as a new thread down in the General Troubleshooting section. However, if you can switch the nameservers to manage all of that through Dreamhost, I think you’ll be much happier with that solution all around.