Navigate to the Manage domains page in the panel, look for that subdomain. Check the web hosting column. It should say "fully hosted" followed by a username. Is that the username you are logging into FTP with? If not, change to this user.
Once logged in, you will see a directory in your home directory named: sub.example.com (your actual sub domain substituted of course). This directory is created automatically when you specify this user in the panel. Change into that directory and upload those two files there.
You can now access sub.example.com/index.html or just sub.example.com/