What kind of export formats does ArchiCAD have? For instance if it can export to stl or obj or 3ds you can import that file into the free program Blender from http://www.blender.org/cms/Home.2.0.html and then export that using the blender exporter into indigo.
atmatt is right. Archicad can export to 3ds and I think also to obj. You could then import your file into blender, apply materials and export to Indigo from there