Product review (4 stars): it's best to pull out the whole rack and remove it ...
This comes with no instructions.  To install, it's best to pull out the whole rack and remove it from the dishwasher.  Place the rack on a table or level surface..  Tilt this roller assembly 45 degrees (between rack wires), line it up, then pivot it to straighten like the other existing ones.  Should install easily.  If you have to force it, you're doing it wrong.  You might consider also buying the end cap stops for for the rack pull-out, since the plastic might break when you unhook it.  Stops Part 8565925.