This is not unusual, if Airfoillabs cannot find your activation key.
If you can... from the Top Menubar, under plugins, find Airfoillabs and click on Product Manager.
You should see: King Air 350, 1.5.3.... and 'Up to date'.
If not, I would delete your installation, re-install and re-activate using the key they provided you.
Then return to the above menu and update the product.