It's a LHS rear axle attachment with one of these;
http://ecx.images-amazon.com/images/I/31dVM1wqNPL.jpg
Which, if you have a solid rear axle, with anti rotation washers, still
fits. Not very elegantly, and I may take the washer off because I think
one will be fine, but still plenty of threads in the nut.
Then you just slip the trailer on and drop in the split pin. Not
actually as painless as it sounds as you have to support the bike. My
old trailer was a high hitch and you could hold the bike by the saddle
while attaching it whereas this one needs the bike to be leant on
something, with not too much lean...