At last, someone get's it! Someone understands how a quality texture should be!
— Andy Molyneaux
I believe you have to create the material files (MXM) yourself using the bmp/jpg material you purchased. The limited Arroway materials that come with Maxwell are already setup for you. Just try and copy the layouts of the existing Maxwell/Arroway files. In fact you should be able to just substitute the appropriate files using an existing MXM from a relative group (Tiles, Boards, etc.)and retitle/rename and save.
Hope this helps.
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