Make sure your computer installed the drivers for the Nook HD+ when you first plugged it in. If they did not install then Calibre will not see it. The easiest way to check is to go to Computer and check your hard drives. The Nook HD+ should show there if the drivers installed. If not try plugging your USB cable into a different usb port. Also, make sure your nook is on and the password entered after plugging the HD+ into your computer. After entering your password you should get the brown pop up, on the Nook HD+, showing it is connected.