Just click ADD DOMAIN in your panel. The next page will as you for domain or subdomain, if memory serves me. Choose subdomain, fully hosted or whatever your account allows. There is a drop down for the domain the subdomain will use...if you are hosting several domains in the same account. Select the one you want, then type in your subdomain name.
Example: Let's say I want a subdomain http://try.zentao.com
I would select zentao.com as the domain I am adding the subdomain to, then type in try in the space provided. Click the submit button, and you'll get a final page that tells you that your subdomain should be up and running in 24 hours or thereabouts. Usually it doesn't take even that long though.
zentao web design, graphic art and design at www.zentao.com
zentao7, Gallery of Artists and Speculative Novel Writers Groups