i've tried numerous ways. i don't think it's listed on the Canadian channel store yet. and adding it directly is impossible with a Canadian Roku account. not sure what's necessary on GB's end but hoping it gets done soon.
From my understanding Roku is kind of bad in Canada. You should use Boxee, as they actually care about their international users. The only downside of Boxee in Canada is that CityTV's website seems to deliberately block the Boxee browser for some dumb reason.