See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.
The LED on the bottom is `C4`. All 7 of the leds are turned on when the keyboard boots-up in the `sixkeyboard.c` file - backlight_enable is not required. The MCU is an Atmega16u2, so the flash memory is limited to 0x3000 bytes - the current setup uses just about all of that! I'm sure things can be opitimised a bit.
There is a jumper on the bottom of the board (next to the USB port) that serves as a reset button - I drilled a hole in my case to allow for quick access via a screwdriver/metal object.