The buckle should be immediately next to one of the other rear seat belt buckles. I can't for the life of me recall which side, but certainly opposite where it starts (I don't sit back there often). I second xpcman, it sounds like the buckle got pushed down between the seat cushion and the seat back.
The middle one has a longer strap to it, so it is easier to loose below the seat. (Strap as in the thing that sits between the mountingbolt and the buckle. I think it is a piece of steel cable for the outer ones and a piece of folded seatbelt-like-material for the middle one) It sits on the drivers side on LHD's.
