The new rule is that your main domain name must be x10 supplied.
You can then add your custom domain as either a parked domain (alias for main domain, same content) or as an add on domian (completely different content from main domain).
Please state the x10 subdomain you wish as your main domain and an Admin will be by to change it for you.
Be sure to remove all email accounts, subdomains, etc associated with the old custom name. The system balks if you try to do it after the custom name is removed.