This is most likely because your network card has no module loaded on the core. You will need to follow some steps to make this work.
Here is one wiki entry that I have added to, but will also get you on the right track.http://wiki.linuxmce.org/index.php/Realtek_8168
also good info here, but lengthy:http://wiki.linuxmce.org/index.php/Unrecognized_NIC
It will give you the info to have the module added to the running kernel on both the Core, and the new MD image.