From Gare du Midi (all international trains including Thalys and Eurostar stop there) you can:

• Take a taxi, which will take approximately 10 minutes and cost less than
  10 Euros.

• Alternatively, use public transport: take a metro Line 2 direction SIMONIS, change at the sixth stop (ART-LOI) to metro line 1A/1B direction STOCKEL/HERRMANN-DEBROUX.  Get off at the next station (MAALBEEK)
  and take exit "Rue de la Loi I". When surfacing, turn left walk down Rue de la Loi. The second street on your left is "Rue d'Arlon": The Renewable Energy House is situated at No 63-67.

From the Airport:

• By bus (Level 0)  Express Bus 12 to "Brussels City".
  
  This is a limited stop service which runs at regular intervals every hour until 20h00.  It stops both at SCHUMAN (roundabout flanked with the Berlaymont building and the pink marble Council of Ministers' Justus Lipsius building) and PLACE DU LUXEMBOURG. Alight at Place du Luxembourg, which is a square dominated by the European Parliament building on one side. Take Rue d’Arlon, on the opposite side. Walk down Rue d’Arlon until you have reached the Renewable Energy House at number 63-67.  At other times take bus No 21 which runs every 20-30 minutes and follows practically the same itinerary, calling at every intermediate stop.
  
  

• By train (Basement Level 1).
  
  There are up to four trains an hour to the centre of Brussels (20 minute journey). Get off at Brussels Gare Centrale and take a metro line 1A/1B direction STOCKEL/HERRMANN-DEBROUX. Get off at the 3^rd stop (MAALBEEK) and take exit "Rue de la Loi I".  When surfacing, turn left walk down Rue de la Loi  for about 300-400m, and take the second street on your left (Rue d’Arlon). Walk down Rue d’Arlon until you have reached the Renewable Energy House at No 63-67.