you can slide it in right at the spout. There's little notches on either side right inside where the lid closes. Try it both ways I often try to put it in backwards and it won't fit. The little point of the U shape of the filter points downwards. :)
The filter goes downwards behind the spout. The top of the filter has little side hooks that slide into tiny grooves on the inside of the spout. My filter goes in very easily. Make sure yours isn't damaged or else it will not attach properly.
There are two little "slots" inside the spout on either side....if you look on the filter you'll see what looks like little on hooks on either side of it. These hooks slide down inside the slots. Will only slide in one way.
To tell you the truth, I have no idea. I think it had to do with those notches. But I put it into a drawer and have never used it for anything. I read the book from front to back and never found an answer either. So sorry.