If you have bought Stack lifetime PRO from the landing page, in order to activate it, you need to sign in Stack using the same e-mail.
You can authorize in Stack by clicking the head-shaped icon on the bottom left corner if you are using a vertical spacebar.
OR on the bottom right corner if you are using horizontal spacebar:
P.s. The information given above may be irrelevant or outdated. We will be updating the page soon. Sorry for the inconvenience and thank you for your patience!