There are 660 parking spaces right underneath SQUARE. You can get in via Place de la Justice-Gerechtsplein, and Stuiversstraat-rue des Sols, and then walk straight into the building.
SQUARE is just across the way from Brussels Central railway station, with national and international high-speed connections to London, Paris, Amsterdam, Köln etc.
Brussels International Airport is only 17 minutes away from the city centre and SQUARE. A direct train shuttle from the airport stops at Brussels Central, the railway station in front of the SQUARE building.