How do I teach my kitten to use a litterbox cat-flap?

I just got my kitten a brand-new covered litterbox with a flap in it. To my horror I came home and he had pooped on th floor. found out that the flap was the problem. I took it off and he is happy. but now the smell is escaping through the door instead of the charcol filter and it is smell. I have tried treats, pushing, coaxing, everything. he will not go through it. help!!