MacFuse provides everything you need to mount an ftp directory on a remote site on your Mac:
- Download and install MacFuse
- Download the Mac port of CurlFtpFs and put it in a directory in your search path
- Create directory ftp
- Execute command
curlftpfs ftp://user:firstname.lastname@example.org/ ftp
The mount point will become visible in the finder. Replace user/pass by your usercode/password and ftp.example.com by the name of your ftp site.
Update: See also MacFusion for a GUI to MacFuse.