Ok now i understand. Well, there is an easy way to do it, but it is only available for OS X 10.7and previous version, anyway here you have the link because they will support 10.8.* in a near future.
https://github.com/halo/LinkLiar
Now, the "not so easy" way depends on your experience with terminal consoles, but anyway it is easy to do it.
Follow this steps:
1- Open System Preferences
2- Open Network
3- Click on your Ethernet connection
4- Click on the Advanced... button
5- At the bottom of the window you will see your MAC address (Ethernet Address: xx:xx:xx:xx:xx:xx) write it down in a text file and save it, just in case you need to set it back.
6- Open a Terminal window
7- Login as superuser (root) in your terminal by typing:
sudo -s
If you didn't set a password for your root user then type your user password
8- Now that you are a superuser type:
ifconfig en0 | grep ether
I am assuming that your ethernet interface is en0 because this is the Macbook Air discussion forum and that's the default interface name for the WiFi interface. The output from that command must give you a MAC address matching the one you wrote down in step 5
If you are not sure what is your ethernet interface name just type:
ifconfig
and you will have a list of your interfaces. For me, when i connect my USB LAn adapter it is en2, you must check yours, and then just change everything that says en0 in these steps to your interface name.
9- Now turn off your ethernet interface by typing:
ifconfig en0 down
10- And now let's change the MAC address by typing:
ifconfig en0 ether your:new:mac:address
11- Turn back on your ethernet interface by typing:
ifconfig en0 up
12- Check that you have your desired MAC address:
ifconfig en0 | grep ether
And that's all
I'm not sure if this change will be permanent or if your MAC address will be rolled back to default after reboot. But now you now how to check it.
Good luck
