There's a thread especially designed for this question.
IT's here:
http://woafor.wacken.com/showthread.php?t=122556
But from the surroundings of Roeselare there's also a bus going.
RTP
Pachthofstraat 146
B-9800 Deinze
+32 9 380 45 05
rtp[at]belgacom.net
I think Deinze is closest by to Roeselare. Otherwise you'd have to go to Berchem (Antwerp) by train first (
www.b-rail.be) and then get on a bus.
Possibilities enough I'd say.
By train:
Roeselare (Roulers) => Kortrijk (Courtrai)
Kortrijk => Bruxelles Nord/Midi/Brussel Noord/Zuid.
BXL => Köln (Cologne) Hbf.
Köln => Hamburg (Hbf/Altona)
Hamburg => Itzehoe.
Avoid Thalys if you can, and take ICE Brussels-Cologne. Thalys is relatively expensive and does not allow "other" tickets but their own.