[anyplace I've used ALL CAPS you substitute your information.]
First is your ftp user assigned to a domain? Navigate to manage domains in the panel, find a fully hosted domain. In the web hosting column on that page, you will see "Fully hosted / User: USERNAME"
You must log into ftp/sftp with that USERNAME.
When you log into that username via ftp/sftp, you should be in the directory /home/USERNAME/ and inside that directory you should find a directory name YOURDOMAIN.COM -- Switch into the directory YOURDOMAIN.COM -- Upload here.
See also these links: