I do not know much about Sculptris, but I'm pretty sure you cannot export a .mesh file in it without a script. You'll most likely need to export it as a .obj (Wavefront object), import it into Blender 2.49, and export it as a .mesh. For you to be able to export it, you'll need this stuff. You'll need the EXACT versions, so no other versions of Blender or Python will work. Here's a wonderful tutorial on it and what you need to install. All you'll need to look at for now is part 1 (I. Required Downloads & Installation) -click- (http://xruby1234x.deviantart.com/art/How-to-Make-Impressive-Title-Items-356893546)
Still confused and need help? Feel free to post here. I can collect screenshots if needed.