if the seat's removed, if you hold up the seat there should be two metal clips attached to it.
On top of the the rear fender, there is a metal piece that has two hook-like protrusions and it should have springs attached it it the keep it in place. When you put the seat on, the clips on the seat push back the metal piece and then it springs back into place and the two hook-like protrusions go through the clips and hold the seat in place
On the side of the bike is a key-hole and turning this will pull back the hook-piece which allows you to remove the seat.
Hopefully this made sense. I can take some pics of my bike later if you still need help