Description on how to find I.P. address on Mac OS X 10.5.x
MAC OSX - Finding I.P. and MAC address for the ethernet adapter
Go to the Apple menu and click System Preferences.
This brings up System Preferences, as shown below. Click on Network
The Network preferences pane is shown below. Make sure that Ethernet is selected and note the IP address on the right panel. Click on Advanced to bring up more information about the Ethernet connection
Click on the Ethernet tab to bring up the panel shown below. Note the Ethernet ID. This is the MAC address for your computer's Ethernet adapter.
Alternative Method
In spotlight search for Terminal
Enter the command "ifconfig -l" to list your network adapters
Pick the network adapter you use (most likely en0) and type "ifconfig en0"
Your IP will be listed next to "inet" .Your MAC will be listed next to "ether"