If your server supports the CHMOD site command then you can invoke it using:
myFTPConnection.InvokeSiteCommand("CHMOD", "755");
Particular SITE commands are not part of the standard, so you should check your sever documentation to see if that command is supported on your server. It's one of the more common commands, so there's a fair chance that it'll be supported. However, even if it isn't supported there may be another command that achieves the same result.
- Hans (EnterpriseDT)