In order to register you need to fill in a document that you can find on the Belgian Interior Ministry's website and return it to your local municipal administration by 28 February at the latest! The form is called 'formulier C/1' and you can download it via this link (see section 4).
EU-foreigners can vote here if they meet four main conditions.
1. You have to be a citizen of an EU country.
2. You have to have your domicile in Belgium and be registered with your local municipality as an inhabitant or feature on the aliens' register of the municipality where you live. This must be the case on 1 March.
3. You must be at least 18 years of age on 25 May 2014 and may not have lost your right to vote in your country of origin.
4. You also have to register for the electoral register in your municipality. See link above.
After you have returned the registration form, you will receive notification that you have been recognised as a voter. You must vote in the municipality where you have registered. You will no longer be able to vote in your homeland. People who registered on a previous occasion need do nothing and will be invited to the poll, if they still meet all the conditions.
Voting is compulsory in Belgium! If you feature on the electoral register you are obliged to attend a polling polling place or vote by proxy.
The Belgian Interior Ministry has also published an interesting leaflet about who can register. It is available in English, Italian, Spanish, Portuguese, Greek, Polish and Romanian.