You can only create subdomains with us if you use the Dynadot DNS name server setting. Otherwise, you will need to set up your subdomains with your web host. Subdomains are also known as subhosts or A Records.
Steps on how to set your domains to use our Dynadot DNS setting can be found here:
How do I set up DNS for my domain? http://www.dynadot.com/help/question.html?aid=4
If you have webhosting with another company, you will need to create the subdomain through your webhost, not us. Our own webhosting plans currently do not support subdomains.
If you have your own web server, you can create subdomains using our Dynadot DNS name server option.