You need Canadian Presence. To be a business in Canada and be eligable to register .ca the business needs to be incorporated. Similar to an american L.L.C. To have it done for you it can cost hundreds by a small time service, and for the most part (depending on the province) one or two thousand. Doing your own legwork can be in the low hundreds, but even for a Canadian the legwork is something of a pain in the butt.
There are alot of domainers here who appear to hold domains for non-canadians, but that's very risky. Not just for the holder, but for the holdee (is that a word ? ).
Good Luck,
Kevin