iMac Remote Lock Wrong Passcode

I was running Maverick on a 2007 iMac 20".

Was updating iTunes and it asked if I wanted to turn on Find My Phone for this mac and I said yes.

Then received a Find My Phone email saying my iMac is being erased.

Received a message to allow a security passcode which i accepted

My iMac then went into a padlock on a grey screen asking me to Enter your system lock pincode to unlock this mac. I did and got an wrong passcode try again.

I tried the security code sent in the message, Same message. Another test code came through on my phone... I entered that and same message.

How can I unlock.

Any ideas appreciated.