click the Please Read Me. link in the left of the window.
This will take you a url where you can download and install the helper app to fully enable WiFiSpoof.
Ok,
The MAC address displayed in system preferences is your Hardware address - and so it never changes. However WiFiSpoof changes the software address, making it completely reversible.
you can check if it really has been changed by tying this command into Terminal.app:
ifconfig en0 |grep ether
the software address is the MAC Address that is broadcast from your computer to external devices.