Yeah, Webs.com is a website builder so you probably don't have the option of creating subdomains. I would research it but their website isn't even working at the moment.
Is your domain with Webs.com or is it managed by a domain name registrar like Godaddy? With Godaddy, you can follow the directions here to create subdomains.
This is why I have and will continue to offer fair warnings on my blog about website builders. They are useful for many people but you need to understand their limitations before you start throwing money at them.