Well, so you have no idea how to unlock it.. okay.
Not sure why you didn't Google it up already, but..
Kind of "official guide":
http://en.miui.com/thread-246705-1-1.html
Another good one:
http://en.miui.com/thread-361618-1-1.html
Some advices:
Hope you already have a Xiaomi account and you already logged into the phone.
I think you missed this beause the Unlocker application says that it indeed sees your device but it is not bound to the account you used when loggin into the Unlocker application.
- activate developer options (Settings - about - tap 5..8 times on MIUI version)
- go to developer options and enable OEM unlocking (Settings - Additional settings - Developer options - enable OEM unlocking) and login with your Mi account
- in developer options enable USB Debugging as well
- Go to Xiaomi security page here
https://account.xiaomi.com/pass/auth/security/home
You must fill everything here and get 100 points - you shall not pass without an account with 100 points.
- Go to
http://en.miui.com/unlock/ and login with the Mi account I mentioned above
- When you apply for the unlocking and have to choose a name write only one word and no special chars like öüóőúéáűí, no $, no ß, nor anything like ?,;.:><[]Đ@&ä or something like these. Just your given name, or nick..
- When you apply for the unlocking and have to type some justification just enter something like "I would like to participate in a custom ROM development and it is crucial to have unlocked bootloader to install our ROM via custom recovery."
- To get the unlocking permission you must to use the phone for 320 hours (AFAIK) before Xiaomi give you the permission (so it's activated).
Hope this helps.
If I missed anything or I'm wrong: peopel, please [FIXME].