Ok that clears up that question. You have to setup a different site. I dont have a cpanel. My host is directadmin. When you add another domain to a plan, it essentially creates a new site under the same parent plan. Plus you would need to create another database. In my case just added a domain alias / pointer since the site shares same code / content etc.
Have you done all this? And need to clarify if you just copied existing silverstripe files into a sub folder in same location?