1) In the app, log out by going to the User Menu using the top right icon and clicking "Logout"
2) Go to the "PLUS" tab and click "Get Instant Access Now"
3) Click "Restore" on the top right of the screen.
4) Select the account with which you registered. Make sure you select the correct account, otherwise you may have created an account that is registered but not subscribed to +PLUS. Please note: you can have both an iCloud account and a non-iCloud account with the same email.
5) If the account you select is not subscribed, repeat steps until you find the correct account.