So far it sounds like they did not upload the files into the “Document Root”, the directory in which the web server starts looking for web page files. The document root is usually a subdirectory within the user’s home directory.
Log into the Web Admin Panel and click on “Domains” then “Web”. Find the subdomain and click on “Edit”. Make sure the domain is set to “fully hosted” and note the “Web directory” settings. For my account it looks like “/home/username/subdomain”. That directory is where the files need to be.