For some tasks you have to know the hardware address (also called MAC address) of your Mac. These documents shows you one way how to find it.
Go to the Apple-Menu, and select the menu item 'About This Mac'.
In the 'About This Mac' window click on the "System Report..." button.
In the left pane of the System Report window click on the 'Network' entry (you might have to scroll down to see the 'Network' entry.
You now should see a list of your network interfaces in the upper right pane. Select the interface for which you want to know the MAC address.
In the lower right pane you should now see detailled information about the selected interface. In the example above I have selected the 'Airport' interface (WiFi).
Look for the text line starting with 'MAC Address:', the address has the form 'd4:3a:00:23:fe:7b'. Be careful not to confuse the MAC address of your Mac with the MAC address of the next router, listed under 'ARPResolvedHardwareAddress:'
Below is another example, this time for the ethernet interface of the same machine (not connected when the System Information app was started):
Here the same interface, this time connected: