I have the MCD40 but I'm guessing it's going to be very similar. Could be a couple of reasons i know of:
The cartridge won't eject unless the changer is plugged into the car as it's an electric mechanism.
However mine has a release button for when it isn't plugged in. It's on the back left of the bottom (when horizontal). It's a small slit in the metal casing about 5mm wide and 2cm long. With mine you have to put a screwdriver in the slot and push the metal plate inside the slit towards the centre of the changer. If it all works as it should it will jump up so you can pull it out.
If this doesn't work look down the side of the cartridge (only a very small gap) and see if any of the CD trays are stuck open. If so I'll try to help some more.
Hope this helps, but remember I haven't got exactly the same changer.
Ben
