I'm afraid we need to know which files and folders you wish to download before first.
Have you seen our tutorial about how to upload or download a single file, how to upload or download multiple files and how to get a list of files and folders? These should provide some information to get you started.
Or, if you just wish to download all server files and folders, call the GetFiles method with "*" argument:
// create client, connect and log in
Ftp client = new Ftp();
// download all files to the specified local path (such as "c:\data\ftp")
client.GetFiles("*", localPath, FtpBatchTransferOptions.Recursive);