Unlocking Pro Features

Plugin: Countdown, Coming Soon, Maintenance – Countdown & Clock

I’ve recently upgraded to the Pro licence to get more customisable features but I am struggling to get the Pro features to unlock in the cms. I have added the site under the ‘manage sites’ tab in my Countdown account and I can see it supplies an activation key but can’t see where to add this to activate it on our site?

Any help would be appreciated.