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Once you introduced Kitty to the Pet Door, put him on the opposite side of the pet door and rattle his food bowl, run the can opener, shake the bag of crunchy cat food... what ever sound he responds to when you feed him.
Place some food near the pet door, so he can smell it. After he starts talking to let him in, pop open the pet door so he can see there is an opening for him to get to his food.
Always give him plenty of praise and affection for doing the things you want him to do during this early phase of training.
Don't use corrective measures-of any kind-in reference to your Kitty's reluctance to use the Pet Door. This is one instance where positive re-inforcement for good behaviors will go much farther to acheiving positive results.