If you want the default shell icon I believe you just leave that box under default icon blank. I'm not sure but I don't think it will accept just shell32.dll.......I believe it should display more like shell32_##.ico
Try this....
use the browse button
set to "All Files" in Files of type
browse to shell32.dll
select icon you wish to use
the program will copy that icon into the \objectbar\skin name\bars\icons\imports directory and give it a name like shell32_12.ico or shell32_15.ico