Good call @2412 about Android devices or alternatively use a PC using a Chrome browser in attempting in redeeming the NowTV offer from your Chromecast.
More information here.
Check out this URL, maybe this solution will work for you like it worked for me, as well as another forum member:-
I've just spent 2 hours on the phone with Google and in a live chat window with NOW, as the voucher code from buying the chromecast did not, as the website say, automatically apply and I've been charged for the last two months. I managed to get the code from the URL and apply it, but of course that means the trial starts now. NOW offered one month free to apologise for the hassle, but won't refund the two months I'd paid for as I'd watched the channels. Erm. I watched the channels because I thought I was getting them for free. I'd logged onto my account to cancel.
Online chat couldn't resolve and now it's being escalated. Asked again if they could give me a month's free pass for the hassle, I said only if that didn't preclude me from getting a refund in the future. They didn't refute this and didn't offer it again or give me the code. Shocking really.
The offer is still there until end of month. See if it works for you. If it does then it wasn't appied properly first time around. You can get the two free months now. Would love to have taken advantage myself but I applied a 4 month cinema pass in January (and have another to use by May)
According to the small print if your chromecast is not installed & up and running by 31st January the offer is not valid for the free 2 months.
My chromecast arrived on the 30th but i did not install and set up until this week and when i tried to get free 2 months it never came up as an option.
I wonder how many people have bought one and been caught out like me.
Crafty move by Google methinks.