The Heysel Palace it's a bit like a Rome of our own: all the roads lead to it. And one can get there which ever way he likes.
Off course, by car. For those that come from outside town, the bypass is only a stone throw from the Heysel plateau: exit 7a "Expo" (Exhibition). From the town, it is obviously a little bit more complicated, but all in all, the sign posts are well indicated, and it will be easy for you to recognise the Exhibition Palace's logo. Once there, there should not be much difficulty in finding a place to park, as long as you are ok to spend some of your hard earned cash.
By public transport, from the town centre, you take the line 6, direct in a few minutes. Go off at the Heysel or Roi Baudouin's stops according to the exhibition hall you want to go to. From the North West outside Brussels areas, the buses number 84 and 89 are good (Roi Baudouin-Amandiers stop). For those that come from the East, take the tram number 23 (Heysel stop). As to the tram number 81, apart from the Jettois, it is better to take the underground. Good luck and bon voyage!