We use our IPad all the time when we travel to the US, pickup a IPad sim from any att&t Corp stores and use your Canadian Amex card to activate the prepaid plan $25.00 for 2 gigs of data.
I allways use the local att&t stores address for the billing info if you do
Not have a American express card then you should be able to get a visa
Gift card and use it to activate.