You will not get data in the USA if data roaming is off. All the Canada-US plan is a billing change, which does not charge you for your use while in the USA.
You need to have data roaming on. Especially because you are an iPhone user. iMessage and other Apple features will not work without data.
I have the same add-on to my plan, which gives me all of the same things my main plan has, but also includes the US. In my case, that's 40GB of high speed data, unlimited Canada to Canada, Canada to US, US to Canada, and US to US calling as well as the same for texting.