If your MCO is one of the old "paper" MCO's (i.e., can't be used for redemption online), and you're based mostly out of an airport that does not have a full service Air Canada desk, then you can call Air Canada reservations. Mention that you're using an old paper MCO and will redeem it and pay for the remainder at the check-in desk at your local airport.

This is what I had to go through at IAH, so writing it down here might save you some trouble. Of course, if you've got an e-MCO, then you're just a few mouse clicks away from redeeming it. But I've found that a lot of the smaller stations will still issue the old fashioned paper MCO's.