If certain controls - like a button or an input field - in an unlock dialog window are not active, even after a product key has been entered, then first try editing the product key to correct any mis-keyed character(s).
NOTE that to activate all required controls in the dialog, the last character of the product key MUST also be re-entered, regardless of whether or not the character was originally entered correctly.
(http://www.sbullet.com/downloads/tmp/Mike/unlockScreen.jpg)
The last digit is generated based on the entire key so it must be re-entered.