The Arduino Uno board on the Mill One uses a slightly modified version of the Grbl 1.1v firmware to run the machine. This firmware (Grbl 1.1v with default settings for the Sienci Mill One*) can be flashed onto your Arduino by using the Arduino IDE software as outlined in the instructions inside the assembly manual.
Grbl 1.1v works best with the latest nightly builds of Universal Gcode Sender, however you can download the most recent stable version of UGS if you prefer.
Strictly for the Sienci Mill One V1:
If you are using an Apple computer with the latest OS (10.12 Sierra), you may experience crashes when plugging in CH340 based Arduino Unos. Please use this driver if you have trouble connecting your Arduino to your computer or if you experience these issues. Windows users should not have this problem. This patch should only be required for the 1st version Mill One and so is not included in the new assembly manual, however the steps to install it can still be found near the end of the Sienci Mill One V1 assembly manual.
*This version is meant for Mill Ones set up for ⅛ microstepping.