When troubleshooting network problems it is often useful to know the comptuer's IP address and MAC address. On Mac OS 10.5.x or newer, you can find the IP and MAC addresses by checking Network Utility.
Open Network Utility by searching for it using Spotlight in the upper right corner or by pressing Command + Space.
In Network Utility, under the Info tab, you will be able to see the Hardware Address (a.k.a. MAC address) and the IP Address under Interface Information
To check the MAC and IP address for your Wi-Fi (a.k.a. AirPort), click on the drop down arrow for Select a network interface for information and select Wi-Fi.
Under Interface information you will be able to view the Hardware Address (MAC Address) and IP address for your Wi-Fi connection