I tried redownloading the checkerboard set, replacing the first try with the second in the props and it still tells me it's not a valid Poser file.  I only clicked the Mac compatible button and if it were only one item I'd call it bad luck and go on but all 12 of the props do the same thing.  I guess I'll have to contact  Tyl2000 and see what I'm missing.  Thanks for the advice.