download a complete folder with its files from ftp server

This is a pretty simple task, solved by using the classes System.Net.FtpWebRequest and System.Net.WebResponse. Your operation of downloading files from a directory would be a compound operation sequencing getting directory lists followed by a number of file downloading operations, optionally in a recursive manner. Those operations are represented by the static class System.Net.WebRequestMethods.Ftp. The operations you need are System.Net.WebRequestMethods.Ftp.DownloadFile and System.Net.WebRequestMethods.Ftp.ListDirectory or System.Net.WebRequestMethods.Ftp.ListDirectoryDetails.

See MSDN help pages with some code samples:
http://msdn.microsoft.com/en-us/library/system.net.ftpwebrequest.aspx[^],
http://msdn.microsoft.com/en-us/library/system.net.ftpwebresponse.aspx[^],
http://msdn.microsoft.com/en-us/library/system.net.webrequestmethods.ftp.aspx[^].

Having all this information and code samples, you can consider the source code you need is almost done.

—SA