http://www.obdev.at/products/avrmacpack/index.html

This site also has tips on how to set it up for use with XCode. I also recommend AVRFuses for determining the best fuse settings and downloading your code using a Mac GUI interface. It uses the avrdude app to do the communication, whihc worked great with my stk500 Atmel board and a keyspan USB to serial dongle. You can get AVRFuses here:

http://www.vonnieda.org/software/avrfuses