Install the addon directly for mozilla's website (link above on the big green button) and let them guide you.
After that simply click the videocamera logo that appeared on your status bar to enable/disable the addon.
To customize IPFuck's behaviour left click on that same logo and select preferences...
To check if it works (it does, but I'm sure you don't trust me !) go to the demo page and hit refresh as many times as you need to realise it actualy works !