You're doing it the correct way. For a new set of curves for a printer, you create a sub-folder for them in:
C:\Program Files (x86)\QuadToneRIP\QuadTone
and they should show up when you restart QTRGui. There's no install routine required as there is in OS X.
I'd like to expand something that datro said, although by the sounds of it you're already aware of it. His screen grab shows the printer selection box, which lists all the installed printers in Windows, but because it's not expanded it doesn't show the separate 'name' and 'printing model' selection boxes. The 'name' drop-down lists all the installed printers in Windows and the 'printing model' dropdown lists all the available sets of curves.
These are sort of independent. On my copy of QTRGui, the available sets of curves is not constrained by the selected printer - it's up to the user to ensure that they've selected a compatible set of curves for the selected printer. Although when you switch from one printer to another, QTR does set a sensible starting point, as described below.
On my copy of QTRGui, I have three sub-folders:
C:\Program Files (x86)\QuadToneRIP\QuadTone\Quad3880
C:\Program Files (x86)\QuadToneRIP\QuadTone\Quad3880-P2
C:\Program Files (x86)\QuadToneRIP\QuadTone\Quad3880-K7
The Quad3880 folder contains the Roy Harrington supplied K3 curves and the others are hopefully obvious. I don't have the Pro inkset, but I assume that were I to install it then I would simply create a Quad3880-Pro sub-folder and place the curves in there, and then in 'printing model' select Quad3880-Pro. I gather this is what you've done, and so you already know that you need admin privileges to make changes in that folder.
There is an extra wrinkle. If in QTRGui you select another printer and then switch back to the 3880, QTRGui defaults to the K3 curves, and you have to remember to select the Pro curves. If this is annoying then you can fix it by renaming the K3 sub-folder to something else, and then renaming the Pro folder to simply Quad3880, and restart the program.
The reason the Pro curves don't appear when you update QTR is because the only curves that the QTR installer installs are the ones that ship with QTR, and my understanding is that IJM now ships the community and Pro editions of the curves separately. To install the IJM curves on Windows, you have to place them in the right place yourself, as above.
Jeremy - perhaps Jeff and I could talk to you offline sometime about your experiences with Pro?