I'm assuming you have an Amiga, and want to transfer a disk image from it, via a floppy disk, onto your Mac. Is that right?
What Amiga and OS do you have? If you have anything since Workbench 2.1, you will have CrossDos built in. You may have to activate it though. If that is the case, you can drag a file named PC0 from your Storage/DosDrivers drawer into your Devs/DosDrivers drawer, and reboot.
You should then format a 720k dos disk, and transfer away. You may need to use lha or similar archiving tool.
Good luck.