You have to flash a dev Rom first.
Then according to the official unlocking thread (
http://en.miui.com/thread-202290-1-1.html) especially this part solved the 50% problem for me:
Kedacom USB device problem (new)
For those who received permission to unlock but still stuck, please check out this section. You might be facing problem due to kedacom USB problem especially for those who are using Windows 10 which causes your PC fail to detect device in fastboot mode. Download Mi Flash Here.
1. Boot into fastboot mode (Hold Volume Down+Power), a Mi bunny image will be shown. Plug your device into the computer.
2. Open Device Manager and search for Kedacom USB Device, double click on Android Bootloader Interface, then click Uninstall, tick Delete Driver Software for this device then OK
3. Unplug and reconnect your phone (phone state is still in fastboot mode)
In this stage, you will see that driver is reinstalling and show Android Device as Android Bootloader Interface, once the driver finished updating it will revert back to Kedacom. The Kedacom driver won't allow you to unlock or perform any kind of fastboot commands. To solve this,
4. Double click on Android Bootloader Interface again under Kedacom again
5. Under 'Driver' tab, click Roll Back Driver, press OK
Since the driver is all right now, head towards Mi Unlock Tool and you should be able to unlock without problem now.
I hope this helps.
Cheers