For Arduino 1.6.1 use this script:
############################################################## atmega328bb.name=ATmega328 on a breadboard (8 MHz internal clock) atmega328bb.upload.tool=avrdude atmega328bb.upload.protocol=stk500 atmega328bb.upload.maximum_size=30720 atmega328bb.upload.speed=57600 atmega328bb.bootloader.tool=avrdude atmega328bb.bootloader.low_fuses=0xE2 atmega328bb.bootloader.high_fuses=0xDA atmega328bb.bootloader.extended_fuses=0x07 atmega328bb.bootloader.path=arduino:atmega atmega328bb.bootloader.file=atmega/ATmegaBOOT_168_atmega328_pro_8MHz.hex atmega328bb.bootloader.unlock_bits=0x3F atmega328bb.bootloader.lock_bits=0x0F atmega328bb.build.mcu=atmega328p atmega328bb.build.f_cpu=8000000L atmega328bb.build.core=arduino:arduino atmega328bb.build.variant=arduino:standard
Otherwise for older versions of Arduino use this:
############################################################## atmega328bb.name=ATmega328 on a breadboard (8 MHz internal clock) atmega328bb.upload.protocol=stk500 atmega328bb.upload.maximum_size=30720 atmega328bb.upload.speed=57600 atmega328bb.bootloader.low_fuses=0xE2 atmega328bb.bootloader.high_fuses=0xDA atmega328bb.bootloader.extended_fuses=0x07 atmega328bb.bootloader.path=arduino:atmega atmega328bb.bootloader.file=ATmegaBOOT_168_atmega328_pro_8MHz.hex atmega328bb.bootloader.unlock_bits=0x3F atmega328bb.bootloader.lock_bits=0x0F atmega328bb.build.mcu=atmega328p atmega328bb.build.f_cpu=8000000L atmega328bb.build.core=arduino:arduino atmega328bb.build.variant=arduino:standard
/Applications/Arduino.app/Contents/Resources/Java/hardware/arduino/
hardware/arduino/
/usr/share/arduino-1.0.5/hardware/arduino
though the version number might be different.