You can only donate in US dollars, so it'll be however much the exchange rate is. If you're doing it via PayPal, it'll automatically convert for you if you're using a Canadian PayPal account and before you confirm the payment, you'll be able to see how much it'll take (in CAD) from your credit card/bank account.
$10 USD through PayPal right now is $10.26 CAD. (And tier 5 would set you back $25.65.) If you're donating through snail mail, you'd have to convert your bills to American anyway so the exchange rate would be given at the bank or exchange office.
That said, now is probably the best time to donate as a Canadian because our dollar is so good these days. n_n;;