If on the stick outer box packing it says somewhere in small print "Redeem during Setup" then the voucher that comes included with the stick is electronically preloaded onto the stick.
Normally once you have setup and activated the stick, the voucher should be automatically saved into the account that you used when you setup the stick.
I would double check on here, by going to My Account (person icon if using a mobile) and then from the drop down menu select Passes & Vouchers then scroll to the top of the page screen to see if there is a message with a link saying something like "Vouchers Waiting for You".
If you see the link, click on it and the 2 month voucher should appear on screen to enable you to redeem and start the voucher.
Should you be on an existing offer with more than 30 days remaining then you won't be able to redeem the voucher until you have 30 days or less left on the current entertainment offer.
If there is no link, then make sure you are signed in on here with the correct account details you used on the stick (open the NowTV App on the stick and go to the My Account section to see what email address or username that you used on the stick).