Get two equal lengths of chain, attach them to the ring on the single tree and then attach one to each side of the sled; it should be very easy to attach those.
However--it would also be very easy to attach shafts to that sled, and that is what I would do. The shafts would have to have to angle up and forward from the bolts on the runners--just typical cutter shafts is what it needs.
On this page:
http://www.summitvie....ca/cutters.htm the original Portland cutter partway down the page shows shafts that would work very well for you.
I personally would not use a little sled like yours without shafts or a pole.