What is the cheapest way to travel from Montreal to Brussel, Belgium during the 2012 Summer ?

There are no intercontinental flights to Brussels South (Charleroi) airport. Brussels has good train connections to Paris, London and Amsterdam (from Amsterdam get off the train in Antwerp). This is the website for international trains by the Belgian traincompany: http://www.b-europe.com/ For Amsterdam and Paris check the Thalys train, for London Eurostar. But since this is the period of the Olympic Games, both flights and traintickets for London will be expensive. The sooner you buy your tickets, the cheaper (but for most train connections you can only order tickets 3 months in advance). Some international tickets include "to any Belgium station" that let you travel for free in Belgium. So compare prices to the different airports. Use sites as http://www.skyscanner.net/ to find the cheapest tickets. And then add the extra cost for the trainticket.

Montreal - New York - Brussels (Brussels Charleroi not Zaventem). All flights by low-cost airline, f.ex. Ryan Air.Then Charleroi -Antwerp by train (45 min.): Tomorrowland is held in Boom (at 15 minutes from Antwerp ) Boom is much easier to reach from Antwerp than from Brussels, but it is your call.