Note: take a backup of the *.qcn file of your own phone using the qpst tool and have it safe , so that you can restore it if anything goes wrong messing your imei or mac or when you want your original mac address back.
Required Tools can be downloaded either in the miui forum thread(Thanks to the thread creator) mentioned in step 1 or through this GDrive link : https://goo.gl/lrTdMb
So lets get started.
1 ) head over to this thread [ http://en.miui.com/thread-348525-1-1.html ]
and follow all the preparation steps(steps 1-8) including installation of software and configuring the phone in diag mode.
2 )Open qpst tool and StartClient-->SoftwareDownload.
then go to Backup and take a backup
3)Open a copy of the resulting file (*.qcn) in any hex editor and search your phone's mac address as hex.
Eg. : if your mac address is 01:02:03:04:05:06 search it as 010203040506
[IMAGE http://i65.tinypic.com/2006xxz.png ]
4)You will get some search results. Now edit every search result with your desired MAC address.
5)And finally restore this edited *.qcn file and reboot your phone once complete.
And that's it. You have successfully spoofed your MAC Address.