Enter the factory code, then within 5 seconds press the 1/2, enter the new code you want with each digit being pressed within 5 seconds each, and you're done. To erase the new code, enter the factory code, then press 1/2 and wait over 6 seconds. That's at least how my 35th is done. Hope that helps.
Don't you also have to press the 1/2 button after you enter the new code. It should cycle the locks confirming the new code. Or is that just on newer Ford vehicles?