When you subscribe to PSEngine, you will receive an email with your license key as well as a set of instructions with links to download the software.
If your email is a bit slow, or you need reference in the future you can find out how to retrieve this data: How do I see all of my available downloads and purchases?
You must download, and install the software on your Windows, MacOS or Linux computer. Once installed, open PSEngine and you will be presented with the Licence Activation Window
Copy and paste your license key in the field as shown below (with no spaces at the end).
Then click Save and then click Activate Licence.
Now click the close button on the licence window to finish. Any time you wish to review or change your licence you can do so via Config > Licence Key.