Apple has apparently removed the GUI support to enable FTP server from OS X. However, you can still enable it from the terminal and can use it the way you like.
To start / enable ftp server in Mac OS X, simply type the following command in the Terminal.
sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist
And to stop / disable it, type the following in the Terminal once again.
sudo -s launchctl unload -w /System/Library/LaunchDaemons/ftp.plist
Hope that helps.