You don't need to do anything to activate Shield. It activates automatically as soon as it finishes installing.
What to expect
Your download was already set up for your account, whether you got it from the Download page in your dashboard or from an install link a Guardian sent you. There's no sign-in step and no activation code to enter on the app itself. The Status page shows your device is protected right away, and it appears in your dashboard's device list too.
If this didn't fix it
If the Status page still doesn't show your device as protected a few minutes after installing, check that your device has an internet connection, then contact our support team.