That happened to me a while ago. Just do this as root: (This is CentOS 6)
nano /etc/udev/rules.d/70-persistent-net.rules and copy the MAC Address of the device you want to setup from the contents of the file.
nano /etc/sysconfig/network-scripts/ifcfg-eth0 (if doesn't exist doesn't matter continue because nano will let you create the file)
3) Insert the next lines:
4) Save and reboot
Now you should have everything working. In any case run the command lspci in order to look all the devices and the ethernet should appear correctly.
If you want, run the command
system-config-network to setup the new device, but if you do this go a check the file
/etc/sysconfig/network-scripts/ifcfg-eth0 again and see that the format is correct, something like this:
If you have any errors post them here and we'll check.