Hi,
The Authorize.net plugin we got just work with USD currency.
If you are sure that your account can deal with CAD, you can update the "authorize.php" file by adding the currency in the list.
class plgHikashoppaymentAuthorize extends hikashopPaymentPlugin
{
var $accepted_currencies = array('USD','CAD');
But please understand that we won't provide any support on the plugin if you have trouble with the CAD currency ; the implementation we made used the US documentation and there was no reference of the CAD currency so I can't provide you any guarantee that it will work.
Regards,