Here is Expatica's guide to help you cut through the red tape on your way to gaining residency in Belgium, including how to register with your municipality.

REGISTER WITH THE MUNICIPALITY AND RESIDENCE PERMITS

European Economic Area (EAA) nationals.
EEA nationals are those from the EU plus Iceland, Liechtenstein, Norway, Monaco and Switzerland. In theory, within eight days, EEA nationals need to register at the local town hall (maison communale/gemeentehuis). In practice, most arrivals have better things to do in these crucial first few days and stretch this timeline a bit.

The documentation you need also varies depending on the commune, your nationality, and frankly the mood of the official on the day. Take the following as indicative and check with the commune before you visit.

As an EEA citizen, you will need the following documents: a valid passport, up to three passport-sized photos, and proof of means of support, usually in the form of a letter from your employer (attestation patronale/verklaring van de werkgever). Some communes may also require birth or marriage certificates.
There are still some restrictions in place for citizens of eight of the ten new EU member states (Estonia, Hungary, Latvia, Lithuania, Czech Republic, Poland, Slovakia and Slovenia). The two-year transition period for the principle of free movement to be fully introduced expired in May 2006. Belgium has decided to lift the restrictions gradually over the next three years, ie, until 2009.

Non EEA nationals- residence permits
Non EEA nationals must apply for a temporary residence permit to a Belgian embassy or consulate before they arrive. Residence permits are restricted to purposes such as study, work (if a work visa is in hand) and family commitments.

Non EEA nationals are then obliged to register at the municipality within three days of arriving in Belgium. In addition to the requirements for EEA nationals, you may also need the following, depending on the commune: up to five photographs, a medical certificate signed by a doctor recognised by the Belgian Embassy, a certificate of good conduct covering the past 5 years (criminal history record) issued by the police authorities of your last country of residence, legalised marriage license (if relevant) and birth certificates for any children. Some communes might take your fingerprints.

There is a small fee, which varies from commune to commune, but it is usually no more than EUR 20, payable in cash. You may well need to speak the language of the commune when you visit: if you don't, go with someone who does. After the first visit, you get a receipt but no card.

What happens next
The municipality contact the police who visit to check that your name is on the door of the address you gave. If their report checks out, EU citizens get an invitation to collect a temporary three months card. This is renewed for a further three months if proof of employment or self-employment and registration with the relevant social security scheme has been or is later produced, and then in due course you get a permanent five year card (carte de sejour/verblijfskaart). Non-EU citizens eventually get an annually renewable  proof of registration for foreigners (Certification d'Inscription dans le Registre des Etrangers/Bewijs van Inschrijving in het Vreemdelingen Register).

Once you get your card, you are obliged to carry it with you since the police can ask you for it at any time. Whenever you move to a new address you must register with the local town hall within two weeks.  The EU carte de sejour/verblijfskaart does not carry the same status as a Belgian national's identity card and is not a substitute for a passport even within the EU.

Here is a list of the major town halls in the communal districts of Brussels. For others see the front section of the Yellow Pages.

Anderlecht        02 558 0800
Auderghem         02 676 4811
Brussels City        02 279 2211
Etterbeek        02 627 2111
Forest             02 370 2211
Hoeilaart        02 658 2840
Ixelles             02 515 6111
Kraainem         02 719 2040
Overijse         02 687 6040
Rhode-Saint-Genese    02 380 2040
Rixensart         02 634 2121
Saint Gilles        02 536 02 11
Saint Josse        02 220 2611
Schaerbeek        02 244 75 11
Tervuren         02 769 2011
Uccle             02 348 6511
Waterloo         02 352 9811
Watermael-Boitsfort     02 674 7411
Wezembeek-Oppem     02 783 1211
Woluwe-Saint-Lambert     02 761 2711
Woluwe-Saint-Pierre     02 773 0511

(expatica Updated 2008)