If you try going into your Desktop Manager and then into the Media Manager you can find out where the ringtone actually is and if it is. If it's on the device memory then transfer it to the computer in a folder you know and then try transfering it back to the mediacard memory on the device.
Now, plug up the BB to the PC, and you should be able to simply use the Media Card as another removable drive on your PC. You can drag and drop files from the PC to the BB Media Card.
Anytime the BB is plugged to the PC, you won't be able to see the Media Card FROM the BB.