SOLVED!
I just don't know how.
I tried the "patch" thing but it told me it couldn't find the right version of DP in the folder I specified (where DP is installed).
So just for kicks, I tried installing the strings again, and Lo, they installed.
Far as I can tell, I did nothing different from the several other times.
I tried them out a bit and they are indeed there.
That said, my initial impression, from listening to 4 or 5 patches is less than overwhelming. they're BIG, for sure, but there seemed to be some level inconsistencies and some reverb--probably natural--that I would prefer not be there.
But, what the heck.
Anyway, thanks for all the suggestions, they at least gave me something to do while the software decided to let me install it.