[quit]never even heard MC6[/quit]

MC6 is the file extension given to "Material Collections". In the Material Room, when you save a file to the Materials library palette, you have the choice to save it as a Single Material (.mt5), or a Material Collection (.mc6). When you distribute the mc6, you also need to distribute any image files (maps) used by the mc6, eg any texture maps or transparency maps that that were applied to the figure when you saved the Material Collection.