I have JRiver MC 18 but other versions should be similar I think, Tools>Options>File Location, in right hand pane under Audio the first option lets you browse to the location you want your music to be stored,in your case D (folder name) something like music or cdrips should do. This will put any cd rips in that folder, then Tools>Options>Library&Folders click on configure auto import and then Add and browse to the folder you created earlier. If you have any music stored elsewhere on your computer put it all in this folder,run auto import and JRiver should find all your music and also should update itself as you add more music to that folder.
Hope this helps if you haven`t worked it out already!!