If you are wanting to customise the page people see when their account has been deactivated, here's what to do:

1) Have root access to your server. You can't access this directory location without it
2) Go to /var/www/vhosts/default/htdocs <-- this is the directory disabled accounts are directed to
3) Customise the files in that directory like you would any other website

If you are wanting to upload files via FTP, just ftp them to a standard hosting account and then copy them across to that directory when you are logged into your server.

Just FYI the skeleton files which are the starting files for any host created in plesk are located in /var/www/vhosts/.skel