@Anonymous User Welcome to the forum. Do you already have an entertainment offer on your account? If so, NOW TV don't let you add a new voucher code/offer until you are in the last month of your current offer. If so, try again when you are in the last month and you should find the apply a voucher link is visible then.
@Anonymous User If your offer runs out tomorrow then you should be able to add your new voucher today. Try popping onto live chat and see if they can see why you can't see the link to add your new voucher. You can get to live chat from the Contact Us page
(expand the Anything Else section at the bottom for options)