If there are any screws holding the back of the door to the front, take them out. You have to prize the two halves apart.
I always use a small bladed screwdriver. (you'll probably need two). I'm always afraid that the plastic will crack, but be careful and it wont. After fitting the catch, just line the two halves up and squeeze together hard. Refit the screws if there were any. You'll probably find it easier if you take the door off the hinge (two screws)
Ray Thomas, March 2005