Tips on How to Train Your Dog or Pet  to Use a Doggie Door:

First - Never push or shove your pet through the opening.  This will scare or frighten him or her and more than likely make them afraid to go through it again.

Second - You can remove the flap or tape it up so that they can freely go through the opening until your pet seems to feel comfortable going in and out the pet doors.  Or you can simply hold the flap open as your pet goes through.  Try holding it open all the way as your pet first passes through the opening then hold it open less and less until he or she gets used to pushing against the flap as they enter and exit.

Third - Praise and reward you pet with a treat as he or she enters and exits through the door.   This works surprisingly well because as you are holding the flap up less and less for your pet and they pass through the opening they are learning to push against the flap more and more to get their treat.