No,you haven't bought a dud, it just doesn't have instructions!
You have already inserted the cartridge and tied it up (if the next bit doesn't work, we'll check that bit is ok).
Open the lid of the bin using the clip on the front (not the two clips on the sides) push the nappy into the liner, then, without closing the lid, grab the white ring part and twist it with your hand. I usually do 2 turns to be sure it is completely twisted on itself. Once twisted, close the lid.
Don't close the lid and twist the circular (clear?) piece inset into the lid, as this has a blade attached which is to cut the plastic "tube" when you want to empty the bin.
To empty the bin, twist the circular piece inset into the lid (mine doesn't work very well anymore, I often use a pair of scissors), then one the two clips on the sides of the bin and upend into a rubbish sack. Tie the now open end of the plastic bag tube, put the lid back onto the bin and do up the clips.
If any of this doesn't Make sense, let me know, I'll watch the thread.