yes, i figured it out under Snow Leopard. First i did the "Open with" method which did not worked and then i choose the "Get Info" dialog, which works.
On a side note, one can also change the meaningless blank paper sheet icon of MoI's .3dm files using this procedure: http://support.apple.com/kb/HT2493
I copied the MoI app icon and all .3dm files show the bulb now. Much better!