From Brussels South Station (Bruxelles-Midi/Brussel-Zuid):
From here you can take metro line 6 in the direction of Roi Baudouin/Koning Boudewijn. This will take you straight to Heysel/Heizel (a journey of about 15 minutes), which is the metro station at Brussels Expo.
From Brussels Central Station:
If you are arriving at Brussels Central Station, you can take take metro line 5 in the direction of Erasme/Erasmus. At Beekkant, change to line 6 in the direction of Roi Baudouin/Koning Boudewijn. This will take you straight to Heysel/Heizel metro station at Brussels Expo. The total journey will be about 20 minutes.