Missing Activate button

While very rare, and because of some security settings on some PCs, the 'Activate' button is missing on the activation prompt:

With this window up, use the Tab key to highlight that area:

Press 'Enter' on your keyboard with it highlighted to proceed.

The other option is to reset your Internet Options so that the button shows henceforth. Directions for that can be found here.

If all efforts fail, activating via the command line is possible.