Assuming you have a separate FTP server as your backup repository then:
1.) Set FTP server to the ip address of your FTP server,
2.) set Path to any folder you created on the FTP server ( this is especially necessary for backing up multiple domains so that you can keep the backup files separate and find them easily ) i.e. "/directory" without the quotes. If you did not make separate directories then the path is "/",
3.) set your ftp login,
4.) set you ftp password.
If you do not have one, the backup to the local respository. The only problem with backing up to the local repository is that in case of disk crash, you loose everything.