to my opinion the best way to get there from the airport is to take the train to brussels, jump of in brussels north, take the train to antwerp and from there the bus to boom. maybe there will be a straight connection to tomorrowland at this particular moment, but this a can't foresee at this moment.
this is a "way" i know.
another possibility is to take the train to antwerp, jump off in mechelen and take there the bus to boom.
i hope this will be enough info to you, regards,