You have to go by train, from Eindhoven Station to Tilburg -> Roosendaal -> Essen (which is in Belgium). After that you have to take the train from Essen to Hasselt via Antwerpen Central.
I guess that's the only way by train.
One thing though: I advise you to buy a ticket in Holland to Essen and in Belgium a ticket to Hasselt. Because if you buy the ticket to Hasselt in Holland you have to pay an extra 3,50 euros. That's not that much, but still. But I don't know how much time you'll have on Essen Station to buy a ticket before the train departs (they often leave once per hour, so that's pretty shabby).
Oh and also, when you're a student you'll get a 30% reduction when you travel internationally, but only in Belgium. That's pretty nice, but I don't know if you are a student ^^
Here are the sites which you can plan your travel (the urls suck though):
Holland: http://ns.nl/cs/Satellite/ns2007/en/mainmenu/include/1193053691834/travelers?p=1193053691834
Internationally (not available in English): http://plannerint.b-rail.be/bin/query.exe/nn?L=ns_hispeed
There you go! ^^

It's not so bad getting there from a large station. But last year we found it a right pain knowing which train to get back on. We flew last year, but getting the train from St Pancreas this time (£49ret)

Brilliant festival though, just hope the weather will be as kind as it was last year!