To Unlock Navigraph AIRAC at SimBrief site:
The easiest is to enter your Navigraph account under My Account at SimBrief site. Then select AIRAC and Unlock.
Otherwise from FMS Data Manager, bottom of Addon List Page, drop down box to SimBrief, Copy Access Code to clipboard, then paste into Unlock AIRAC section at My Account at SimBrief site. Then select AIRAC and Unlock.
Cheers
Ian