ShapeShifter and Fusion are both for running Mac programs on an Amiga. To run Amiga programs on a Mac you need UAE check it out at
http://www.freiburg.linux.de/~uae/ . You will also need an Amiga rom file and Amiga OS disk images. They are available
by serching online, but are still copywrighted so you should only use the online images if you have the real ones. Using the kickstart rom images and OS disk images off of the Internet just makes life easier then having to network your Mac and "real" Amiga. If you don't have the rights to use the rom and OS or if you can use some of the nice extras included you may want to consider Amiga Forever.
http://www.amigaforever.com/Hope this helps,
Xerxes Wahl