It seams that flex is unable to receive the MOUSE_WHEEL MouseEvent on Mac computers. This is because Flash MouseWheel is not supported on Mac by default. This has been submitted as an enhancement request already. At the meantime, there are add-on examples you can consider. See one of them here:
http://reefscape.net/?p=13
http://blog.pixelbreaker.com/2006/11/08/flash/swfmacmousewheel/
