Every time I have had that problem (that is, the ethernet card not being found) is has been because the correct module is not loaded.
I googled for your mobo and found that your NIC controller is (probably) this one Realtek 8201 Ethernet Controller. If you type:
$ /sbin/lspci | grep -i ethernet
you will be told exactly which controller is running your onboard NIC. Real-Tek have really good support in linux and you may find that the module you need is already available in your kernel.
Hope this helps.