As some people asked:
Here's how I got my #FOSDEM SIM and how I set it up:
Got off the train from the Airport at Brussels Central.
Left through the southern exit (direction of train running)
Went to the Store on the top left, picked up a free SIM. (Big Lycamobile stickers on the windows)
Went back into the station and bought 10€ worth of top up code from the Carrefour Express shop.
Put the SIM into my phone, disabled mobile data.
Apply top-up: *103*codegoeshere# (press dial)
Activate "Data 10" bundle: *139*3010# (press dial, press 1 to confirm)
reenable mobile data (that was a precaution so that the phone doesn't go online and use up the top up)
There is also a Data5 (3GB) for 5€, not sure if they sell 5€ top-ups though. For details see http://www.lycamobile.be/en/bundle?id=3
#exactsteps