I've learned a few things since posting this ad.
The magnifier, in fact, screws into the eyepiece. To use it on a D90, as I did, I had to get an adapter for it, a
Nikon DK-22. The magnifier screws into the adapter and the adapter slides onto the eyepiece. The magnifier, BTW, has dioptric correction ranging from -5 to +1.
The adapter is included in the sale, and, according to the B&H page, will allow the magnifier to be used on the following cameras:
"
For N2-4-5-6000 Series, N50, N55, N60, N65, N70, N75, N80, Pronea, FE10 & FM10, D100, D70/s, D60 & D50, D3000, D5000, D80, D90, D300 and D300s Cameras."
If anyone thinks it might fit their camera, but is hesitant, please keep in mind that you can return this for a full refund.
Charley