Like Machtelt Garrels points out, the source code for the software used on the voting computers for this year is not yet available. It only is made available to the general public the day of the election after the voting has closed. The government has a FAQ about it here (available in Dutch, French and German).
There are 2 pieces of software that can be used: Jites or Digivote. They're not compatible with each other, so it depends on the municipality you have to vote in which one is being used.
Here you can find a study of Belgian voting system and a comparison with other countries. It's available in Dutch, French and English.
Update: The sources is available before the vote, just not to the general public. It's available to the politcal parties and the parlement can appoint a specialist to inspect the source code.