The '96 Mustang has a separate PATS module. I have seen other NEW programmers that would not communicate with older vehicles. That's why I keep my old DS708 to use with the older vehicles. That thing still works great with my '02 Ranger. You only need one key to add new keys.
Insert an already programmed key in the ignition and cycle to RUN then OFF, Insert key to be programmed and cycle to run, Wait for security light to turn off. Let us know if the above method gets you fixed.