Place the following into /etc/network/interfaces with an editor.
It will cause the network manager to bring up your Ethernet automatically and configure it with DHCP.
auto eth0 iface
iface eth0 inet dhcp
As Morgan correctly suggested you can find out if you should use something other than eth0 to refer to your wired connection by using the command
After adding this restart the network manager with:
sudo /etc/init.d/networking restart
(You should get an error message).
If this doesn't work you can try:
sudo ifdown eth0
sudo ifup eth0
And if that doesn't work you can reboot.
If all this doesn't work for you please mention in your question what you tried and what messages you got. You might find more information at the bottom of
/var/log/syslog if you look right after you try it.
tail -n 50 /var/log/syslog