This is a point of confusion for many no doubt.
When you install CWP you have to associate a user and domain. But if that is the only domain, it doesnt appear in the addon domain list. Normally, when you add on a domain it is stored in /home/myusername/public_html/myaddondomain.com
In reality the main domain is stored at /home/myusername/public_html
So if you have one domain, your main webpage would be /home/myusername/public_html/index.html