This design, including the software and hardware, is licensed under the MIT license.
The PCB design is made in Cadsoft Eagle 7.1 (http://www.cadsoftusa.com/download-eagle/). The free version should be fine for opening and editing the design.
It's based on the Atmel ATTINY85, though an ATTINY45 or ATTINY25 would probably also work.
The firmware is written against avr-gcc.
Install XCode and command line tools. Version 6.1.1 was used for development, but this should not be critical. The method for enabling the command line tools seems to be a moving target, one set of instructions is here (https://developer.apple.com/library/ios/technotes/tn2339/_index.html).
Install CrossPack for AVR (http://www.obdev.at/products/crosspack/download.html). Version CrossPack-AVR 20130212 was used for development. Most likey this code is not version-sensitive.
Once the toolchain is configured, this should be as simple as:
make
An AVR ISP is required to upload code. The Makefile is configured to use a USBTinyISP (https://learn.adafruit.com/usbtinyisp), however it would be trivial to use a different one, or even an Arduino to upload the code.
Solder the 6 ICSP test points on the board (use Eagle to inspect the pinout- the board design is a moving target), then an automatic upload should work:
make install
A test jig can also be built to make code upload possible without soldering.