Yeah, cats will always look for the easiest ways to do things. They are quite lazy animals. If you put some food on the other side of the cat flap, it should speed things along.
I just pushed mine through it (without the flap), once she figured out that it was an in an out, I just put the flap over it. She got the message. Cat's are pretty smart.
All you need to do is show them...once cats see a means of getting out, they'll take advantage of it.
kick them through it :D thats wat i did now she always want to leave :D