Hi - this may be old news but thought I would post here in case anyone is trying to upload grbl to their Arduino. I found a simple solution using Xloader
I did this on a Mac running Windows XP through VMWare. I installed the Arduino IDE to make sure I could communicate with the board (an UNO) and had to screw around a bit setting up the COM port (esp baud rate) - the instructions on the Arduino site are helpful. After that, it was just a matter of launching Xloader, selecting the .hex file and pressing the button to upload.
Afterwards I ran ZTerm (terminal app) on my Mac and was able to get the confirmation msg from grbl and a dump of the setting.
Really painless. Yay!