Enter Your System Lock Pin Code To Unlock The Mac

Are you referring to the iCloud Keychain?

If you don't know and never set a lock pin code then the only people that can help you is Apple. You'll need to take the system into an Apple store and prove ownership.

If by chance you bought your Mac used it might of been locked by the original, Real, owner.

Wow, I was unfamiliar with how this all actually works. The machine has been remotely locked using Find My Mac. The previous owner remotely locked the machine once it was available over Wifi. Your only solution is to contact the previous owner, or go to Apple/ contact support (see the number at the bottom of this page).

I am acting under the reasonable assumption you did not steal this machine and the owner is trying to retrieve it.

Best of luck.

More info:

https://discussions.apple.com/thread/4133010?answerId=19010713022#19010713022

There have been a few reports of users finding the computer locked after upgrading to Mavericks.

Anyone else get locked out of their computer...

Installed Maverick and asked for four digit code