I guess, if you are under Windows, that you have simply set your Nikon D300 to PTP mode. When you connect your camera to computer, it can have one of two modes, called Mass Storage and Picture Transfer Protocol.
PTP is for controlling the camera FROM computer, used by Windows and Nikon Camera Capture, that is, if you want to remotely control the camera and download the photos.
Mass Storage is such that whatever card in the camera, the computer sees it as a normal external drive. That mode is used by all card readers, external hard drives, USB flash sticks et cetera.
Just set the camera to Mass Storage (somewhere in the menus) and voila, Windows should see the camera as just another external disk drive, which then shows correctly in PM.
Or better yet, use a dedicated card reader. They are faster, more convenient and cheap.