I've never heard of anyone being able to write Amiga formatted disks on a Mac or a PC. I think that there is an add-on PCI card or something that you can get called CatWeasle that you put in a PC that allows you to read/write Amiga floppies...
Note that once you get your Amiga up and running you can use tools such as CrossDOS to read / write PC formatted floppies on the Amiga.
