The reason why you can't find much in the way of useful FTP client software is because FTP isn't used much any more and is generally actively discouraged from use.
Without great care, it is quite easy to create big old security holes when using FTP (when I ran a consulting company, the 3 times we had infected machines were all because of FTP server security holes or exploits -- one time, the damned HP copier's FTP server was the attack vector!).
FTP is also inefficient unless carefully configured.
I would encourage you to use an HTTP based protocol. WebDAV allows you to do basically anything FTP can do, but does so over an HTTP channel. Thus, it'll work through proxies and the like. Heck -- HTTP has become so ubiquitous that pretty much everything works with HTTP.
And, of course, there are plenty of good HTTP client APIs built for Objective-C.
Obviously, if FTP is a requirement for your project, this answer won't help you much....