Success at last. I put the hex file in my Ardunio user folder a provided an explicit path in my statement as follows:
/Users/myUserName/Ardunio/build/macosx/dist/tools/avr/bin/avrdude -C/Users/myUserName/Ardunio/build/macosx/dist/tools/avr/etc/avrdude.conf -pm328p -carduino -P/dev/tty.usbmodem411 -D -Uflash:w:/Users/myUserName/Ardunio/grbl_v0_8b_edge_328p_16mhz_9600_build20121013.hex
And got:
Writing | ################################################## | 100% 4.26s
avrdude: 23050 bytes of flash written
avrdude: verifying flash memory against /Users/myUserName/Ardunio/grbl_v0_8b_edge_328p_16mhz_9600_build20121013.hex:
avrdude: load data flash data from input file /Users/myUserName/Ardunio/grbl_v0_8b_edge_328p_16mhz_9600_build20121013.hex:
avrdude: input file /Users/myUserName/Ardunio/grbl_v0_8b_edge_328p_16mhz_9600_build20121013.hex auto detected as Intel Hex
avrdude: input file /Users/myUserName/Ardunio/grbl_v0_8b_edge_328p_16mhz_9600_build20121013.hex contains 23050 bytes
avrdude: reading on-chip flash data:
Reading | ################################################## | 100% 3.32s
avrdude: verifying ...
avrdude: 23050 bytes of flash verified
avrdude: safemode: Fuses OK
avrdude done. Thank you.
I really appreciate the help.