This is because your network interface has not been configured.

run the following to get it working.

dhclient

Assuming your network card gets a DHCP lease you should now be able to perform your yum command.

To ensure that your server boots correctly in future edit:

/etc/sysconfig/networking/ifcfg-eth0

Where eth0 is your default NIC and ensure that

ONBOOT=yes

and

BOOTPROTO=dhcp.