You gotta loosen the bottom and top bolts and the chain tensioners. The whole rear carrier should move quite a bit on the back end. If all that is loose, start pushin and pullin on that carrier bottom to get it to move,. It pivots on the top bolt but everything needs to be loose back there in order to set it all.
When tightening, remember to leave 1-1.5 up and down slack in then chain.