I currently have an HP Proliant DL380 G5 server. My CentOS has no GUI, just CLI. It has two working embedded NIC cards and are appropriately listed as eth0 and eth1. I installed an HP NC380T PCIe Dual Port Multifunction Gigabit Server Adapter and when I enter BIOS the NIC card is detected and both Ethernet ports are listed.
When logged into CentOS as root user, I enter setup from CLI and choose "Network Configuration I go to Edit Device and choose New Device to add Ethernet. Inside Configuration I enter eth2 and I give it a static IP with a Subnet Mask".
I then go into Firewall and choose Customize then I select eth2 as Trusted device as well as Masquerade. I type ifconfig eth2, I get message "error fetching interface information: Device not Found" Please can any one assist me in getting it working?

Are you sure the new NIC came in as eth2?

From command line as root run "ifconfig -a". That should show you all network devices the system thinks it has even if they're not configure yet.

The device files are in /etc/sysconfig/network-scripts as "ifcfg-<dev>". If you see any devices listed in ifconfig output that don't have a file there you may need to create one from scratch on adding a new NIC. You can look at the existing ifcfg-eth0 and ifcfg-eth1 files for ideas. One of the most important things is to find the Hardware/MAC address for the new interface and insure it is in the relevant ifcfg-<dev> (and also insure the same address is not in any others.

You can run dmidecode, lspci and "kudzu -p" on RHEL/CentOS 5.x to get other information about devices in your system.