In order to use PORT mode you'll need to configure your router to accept incoming connections and forward the ports to your computer. You'll need to then set the same port range in FlashFXP. In addition you'll need to tell FlashFXP your external IP address, either by using a dynamic dns address or by using
http://iplookup.flashfxp.com, all of these settings in FlashFXP are set in the Preferences dialog.