Welcome to the forum. If you've bought an entertainment pass from the NOW TV website your pass is started straight away and is ready to watch, voucher codes are used for offers, gift cards and other promotions.
Try watching something on whichever device you use, it should work straight away.
What website did you buy the voucher from?
Passes are applied to your account rather than your device, so if you buy a pass from NOW TV it's good to watch on any device you have registered on your account (you can have up to four devices registered).