Open up a Terminal and paste in the following commands to enable or disable the built-in FTP server.
To turn on the FTP server, paste this into the command prompt:
sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist
To turn off the FTP server, issue the following command:
sudo -s launchctl unload -w /System/Library/LaunchDaemons/ftp.plist
Now you can access your Mac via FTP using a command like this from another computer on the network:
ftp://user@your_mac_ip
ftp://user@your_mac_hostname