https://groups.yahoo.com/neo/groups/QuadtoneRIP/conversations/messages/12903
To spell things out in precise detail, in Windows, the shipped curves are stored in these two folders (using the R2880 as an example):
C:\Program Files (x86)\QuadToneRIP\Profiles\R2880-uc
C:\Program Files (x86)\QuadToneRIP\QuadTone\QuadR2880
The files in the Profiles folders are .qidf files which you can edit to modify the curve. QTR will then "compIle" the curve to create a .quad file. These .quad files are stored in the QuadTone folders. There isn't a .qidf file for every .quad file, which doesn't matter so long as you don't want to modify the curve, all you need to print is the .quad.
You can store additional curves in these folders if you wish. The problem with doing so is that you'll need admin rights to add / delete / edit files. The QTR Curve Creation tools use alternative folders to store user-created curves:
C:\Users\[user]\AppData\Local\VirtualStore\Program Files (x86)\QuadToneRIP
The idea here is that you do have read and write privileges to this folder and its sub-folders, so this is where QTR stores curves created using the Curve Creation tools. I guess it's where you're supposed to store any files that you add yourself. When QTR displays the available curves, it merges the lists in Program Files (x86) and VirtualStore. You don't need to use the VirtualStore for anything you add, you can use Program Files (x86), but you'll need admin rights to do so.
However, in W10, the virtual store is broken for QTR. If you create a curve using the Curve Creation tools, you'll need to initially save them elsewhere and then move them into either Program Files (x86) or VirtualStore.