I found that easiest emulator to use is Atari800MacX...
http://members.cox.net/atarimac/
Just copy your roms in the "ROMS" Folder and there it is.You you still have problems with the roms, just go to the preferences and set the correct directory. I think Atari800MacX is far better emulator than the old Rainbow... At least for a guy like me who just loves old Atari XL -demostuff :)