you got the adjustable webbing suspension i think. you'll have to take the webbing out of the buckles, wrap around the tree and thread one end through the looped end. now webbing is attached to the tree. thread back through the buckles, adjust, and tie a slipped half hitch tightly behind them.
if you get 2 biners, you can put them in those webbing loops, and you just clip the webbing around the tree, rather than taking the webbing out of the buckles, and then threading them back on. this is what most folks do.