The Brussels tram system is in a, seemingly permanent, state of flux. In 2007, the most direct route between the central Grand Place area to Hysel Stadium (the nearest stop to the Atomium) was Tram 81*, however this has little to recommend it as the trams are small and over-crowded, and the route serves some of the rougher areas of the city.
A better bet is Tram 4*, which runs to Esplanade, a short walk across the park, using brand-new high-capacity trams. Between Van Praet and De Wand Tram 4 shares an underground track with Tram 23* which runs to Hysel Stadium and you can change at any of the intervening stations.
True tram addicts (and Iʼd count myself as one) might prefer to catch Tram 23* the whole way between Gare du Midi and Hysel Stadium, a fascinating mix of street running, dedicated tracks and underground (pre-metro) sections.
One and three day unlimited travel tickets are widely available from various outlets around the city.
* As far as I can make out from Sept 2011: Line 81 became Line 51, Line 4 became Line 3 and Line 23 became Line 7. Line 7, however no longer operates to Gare du Midi. Check the STIB site for current information.