The Straßenbahn goes directly from Darmstadt - Luisenplatz. You have to take Nr. 6, 7, or 8 in the direction to Alsbach and get out in Jugenheim, Ludwigstraße. From Ludwigstraße it is quite a walk up to Schloss Heiligenberg, approximitly 20 min - 30 Min, depending in what shape your in :-).
Important! Please let us know some time in advance if you are planing a tour. But as I mentioned earlier on, the Schloss-Park is open to the public. You just walk up there (or drive) and have a walk around. As soon as the new website of the Fondation is finished and online, i will post it for more information.