You have to plug the Stick into your TV and go through the setup process, and then the Pass on the Stick will go onto your account. You may then have to go into your account and Redeem the Pass in My Account/Passes & Vouchers.
If you bought, separately, a physical pass, then you have to scratch off the coating on the code on the Pass, and type that in on the P&V page, somNowTV knows you bought a valid pass.
This should all be explained in the blurb with the Stick, or on the physical pass.
Have you done any or all of this?
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.