In the app please go to "Settings" - "USER SETTINGS", and tap the text "click here to sign in" near "Account".
A new page will pop up, where you'll have to tap the button that says "Sign in with Apple" (iOS) or "Select Google account" (Android). On iOS you can hide your real email from the app, it won't matter. When you're logged in, your Account ID will be displayed on the Settings screen near "Account".
Tap your Account ID, and you'll see an input field where you can enter your activation code. You can also paste the code from the clipboard by tapping the document icon below the field. After inserting the code, tap the green checkmark and your code will be validated.
If everything is OK, the FULL version will be activated for your account, and you can use it on any iOS or Android device where you log in with the same account.