On a terminal window type:
% sudo ifconfig <interface> ether xx:xx:xx:xx:xx:xx
where <interface> is your NIC (normally en0) and xx:xx:xx:xx:xx:xx is the new MAC address that you want to set.
On a terminal window type:
% sudo ifconfig <interface> ether xx:xx:xx:xx:xx:xx
where <interface> is your NIC (normally en0) and xx:xx:xx:xx:xx:xx is the new MAC address that you want to set.