Altervista doesn't have FTP options at first glance but, if you use Filezilla or something similar, you can use that if you'd rather work outside of the dash (like if you need to DL the whole site for a backup).
To log in via an FTP client, you'll need
HOST: yourusername.altervista.org USERNAME: yourusername PASSWORD: The same pass you use on the site. PORT: 21