The the lock nut (castle nut) SHOULD NOT BE LOOSE. It should be tightened down holding the end plate tight against the reciever where the indentation on the rear of the receiver prevents it from turning.
First insert the retainer and spring into the the lower reciever (the hole in the picture above). Use your finger to hold it into place as your twist the buffer tube until it goes over the top of the retainer pin (but not past the nipple in the middle).
Then make sure the detent and spring for your rear takedown pin is in place (it goes in a hole in the rear of the receiver) and slide the endplate down the buffer tube to it pushed the spring into the hole and fits into the indentation on the back of the reciever.
Then hand tighten the castle nut as tight as possible. Finish it off with stock wrench. You really should stake the nut in place. But if you are like me and like to change stuff around or are just a little lazy, make some marks with a sharpie indexing the position of the castle nut to the end plate. Check it periodically, like when you lube ur bcg, or at the start of a shoot or when you take it down for clean.
You don't need another retainer or spring, but if Bushy's already sending you one, it never hurts to have spare. Becareful tho, that little retainer might just grown into a new ALLOY AR15 someday....