The schematic is like this:
switches leds
,--+--+--. ,--+--+--.
|C7|B7|B5| |C6|B6|B4|
+--+--+--+ +--+--+--+
|D6|D1|D4| |D5|D2|D3|
`--+--+--' `--+--+--'
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.