1. Hey I'm in the same boat. I have successfully downloaded GRBL though. Then saved into my Arduino IDE/Libraries. Next, I downloaded g-code-sender. I then followed a tutorial on connecting GRBL to g-code-sender then it was installed into my UNO. Finally I could upload a sample g-code to my UNO from g-code-sender and see the RX/TX leds flicker to let me know its working along with watching each line of code execute on the g-code-sender program. P.S. I'm on Windows7 (64bit).
2. I have been going through the config.h(grbl) and I can't seem to "REPIN IT", if you will, to accommodate the Arduino UNO?Ada-fruit Motor Shield. I believe the GRBL program will supply direct to a Stepper Driver and not to a Translator(sn74hc595n on the motor shield). To my understanding is that The Motor Shield works like Co-Phasing one signal into two, that's where the Dual H-Bridge comes in. I am self taught and not even a noob yet(16yrs on/off) and I hope this helps somewhat.
I will release more info as I have it.
Take Care, DoUduino