I've used an eMCO before.
Some stipulations in case you did not read the brochure properly:
1. The voucher only covers the base fare.
2. You're on the hook for the difference of the flight and
all taxes.
There are several ways to use the eMCO. The best and fastest way is to call in to reservations or visiting the ticket sales desk at your nearest airport.
There are other ways to use the eMCO, but not very convenient. The worst is - You can't use it for bookings on AirCanada.com.
Sometimes, free has its catches. If you travel often and know you will go somewhere, they're great. But for non-frequent fliers, it's a pain. You could have opted for cash instead at the time of volunteering (but you knew that already.)
I personally recommend using the eMCO towards a Flex, Latitude, or Business Class flight if you're into collecting qualifying miles. In the end, AC has the last laugh. Good luck on the voucher and use it wisely.