I'm following the instructions for the lower receiver installation as given on this site, but I don't quite understand where to install the retainer spring and retainer.  There's a recess which I believe is where it goes, but I don't know how far to screw in the stock and then how this retainer "retains" the buffer spring and buffer.

Any help would be appreciated, there's nothing that's keeping the retainer pin from just being ejected by the retainer spring.

The retainer/spring goes in the recess you describe, You have to hold it down and screw in the buffer tube till it makes contact with the shoulder of the retainer, The buffer tube it's self holds the retainer in place.
Good Luck!

haha thanks a ton, I figured it out just as I posted

The front edge of the buffer tube is screwed in far enough to catch the rear edge of the buffer retainer and hold it in place.  You didn't indicate whether you're building a full stock version or a fake folding stock piece.  On the full stock version tube, when the tube is screwed all the way in, that front edge will overlap the hole for the retainer enough to hold it.  On the fake sliding stock piece, you're going to have to manually screw it in just far enough so the front edge overlaps the edge of the hole about one third of the way, and then tighten up the rear ring.

Obviously, you'l have to install and depress the retainer and spring while you're screwing in the tube.

(Edited to add)[;D]It's hell being a two finger typer. I'm always two minutes behind the other guy in getting my answer posted[:D]

That's why it only took me half the time, I only use one (honest!) [:D]