* Spoof the MAC address:
sudo ifconfig en1 ether [mac address to spoof]
Where it says "[mac address to spoof]", press Command-V to paste the victim's, er, I mean the machine-to-be-spoofed's MAC address. Your machine may require an administrator password at this point. This is the password you use to log into OS X on your own machine.
* Confirm the spoof:
ifconfig en1 | grep ether
Your displayed MAC address should now be different from what it was before.