Never experienced a DimPro failure where the files actually existed but could not be found. Does the stand alone version work? Could the permissions have been changed on the folder making the files unreachable?
Make sure you are looking in the correct registry entry in
HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\dp key "Multisamples Folder"
The last resort would be the clean reinstall suggested in post #2