They seem to offer proxies that support multiple protocols at once and recommend using SOCKS5 (see
http://advanced.name/faq). To connect to a SOCKS5 proxy using Rebex FTP/SSL, use the following code:
var ftp = new Ftp();
ftp.Proxy.ProxyType = FtpProxyType.Socks5;
ftp.Proxy.Host = "1.2.3.4"; // replace with proxy address
ftp.Proxy.Port = 1080; // replace with proxy port
//ftp.Proxy.UserName = "user01"; // use your user name
//ftp.Proxy.Password = "password"; // use your password
ftp.Connect("test.rebex.cz", SslMode.Explicit); // use the desired host name and SSL mode
However, I have been unable to try this with their free proxies - most of them seem to prohibit FTP (see the note at the end of
http://advanced.name/freeproxy).