To use your FTP Backups you will need to create a new account within your FTP client.
The details you will require are:
FTP Host: ftp-backup1.donhost.co.uk
Username: Your FTP backup account username
Password: Your FTP backup account password
You can check your FTP backup account usernames and passwords via the Manage Backups screen in the Donhost Control Panel. This article (How do I manage my FTP Backups) will assist you in accessing this page.