@Tony2004 , you'll first need to apply a top up to the balance in the amount needed to pay for such an add-on. You can purchase U.S roaming add-ons from your self service online account.
Do you want data for use in USA ? If so, look up for Plans, then scroll down til you see Add-ons (International)...find the 10 Day USA Roaming Data Add-on that you want there. You'll go into your account and pay first, then go back to actually buy the data plan. It's only good for 10 days and is active as soon as you buy it.
Are you asking to buy US roaming data? or Canada data? Unable to buy international roaming data with Public Mobile.
Here is a good thread about buying add on:
1) in self-serve click Purchase Add-ons and select an add-on. It will then want you to add money even though it looks like you're buying an add-on. You can use the registered payment card or enter whatever payment card or a pre-purchased voucher. Add the money then go back and buy the add-on with the money you just added.
2) call the 611 service and the same...either the pre-registered payment card or voucher (not just adding any card though).
Edit: ninja'd. I'm getting too slow at this
Edit: ninja'd just for that!
If you hadn't gone for the link I'd be the one writing that. :-)