From the picture I cannot see how this mounts on the carbine rear sight. Can you help?
This rail attaches by first removing the rear iron sight, then installs in the place of the original rear sight.
The rail will line up with a dovetail located on the receiver. If it does not want to go on then the rail may require a bit of filing. NcSTAR parts tend to be very roughly finished and lack the low tolerance specs of other parts that sell for several times as much, as such you may need to sand or file an edge here, grind an edge there, etc. to get some of their stuff to fit properly.
Once you have a proper fit use a mallet to make sure it's properly seated then insert the base screw from the original rear sight and tighten it down.
If the rail does not seem flush then take it off and file it down until it seats to your satisfaction.
After using the firearm you'll want to tighten down the mounting screw again.