This is getting repetitive, but like the other two operating systems, the quickest way to find your MAC address in Mac OS X is to use the command line. Open the Terminal application using the Finder or Spotlight search, and type ifconfig at the command line. The output isn't incredibly easy to read, but your MAC address should be listed as ether, as shown below.
To find your MAC address using the GUI, open System Preferences and then Network.
If you're on a wired connection, click Built-in Ethernet and then click the Configure... button. (I also want to note how much more sense this terminology makes than the equivalent Windows naming convention.) The Ethernet tab contains your MAC address, which can be selected and copied to paste elsewhere.