DHCP is the opposite of manual configuration. To enable DHCP, if it isn't already, open the file /etc/rc.d/rc.inet1.conf. In the section under the eth0 interface there is an option DHCP="xx". If you want it then put yes, otherwise put no. If it's yes it'll try to set up the eth0 interface at every boot, wether you want it to or not. If you set it no you can still enact the DHCP process from the commandlin by issuing
and wait a few seconds. If you want to set IP addresses manually (static) you can just set them in the same rc.inet1.conf file and it'll be configured with that address at every boot. Don't forget the netmask and gateway.