"not managed" means that network manager is not responsible for your interface.
I assume you have a DHCP server in your network
You will have to edit /etc/network/interfaces to look like
(lines starting with a # are just comments, the important point is that no configuration for eth0 except 'auto' is stated)Code:# The loopback network interface auto lo iface lo inet loopback # The primary network interface auto eth0 #iface eth0 inet dhcp
Then restart network manager