I had the same problem and solved it like this:
Open control panel - Open Fences 2.0 - Press "Purchase Fences" - Press "Reactivate previous purchase" - Enter login-information and activation code (where it says "serial number") - Press "OK" and wait for the program to be activated.
It does not seem to be the right way around and I fail to understand, why there is no "Enter license key"-button to be found anywhere in Fences, but it worked for me...
BR
Erik