I have to admit that I've never joined a Mac OS client myself. However, when joining a Windows machine you do not create a computer object in the domain first. Instead such an object is created automatically during the join process.
So please remove your newly-created computer object from the domain. Then try joining once more. Also try "administrator", "domainnamen\administrator" (replace "domainname" with your actual domain name, of course – the short one) and "email@example.com" as the user name when joining. For example, our FQDN is bs.linet-services.de, and our short (NetBIOS) domain name is "linet". I'd therefore try "linet\administrator" and "firstname.lastname@example.org".