The GUI for FTP server has been removed in Mac OS X 10.7, however you can still enable it from the command line.
To start it type the following command in Terminal.app:
sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist
To stop it, run this command in Terminal.app:
sudo -s launchctl unload -w /System/Library/LaunchDaemons/ftp.plist
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