Qucik2Wire made and marketed some add-on boards for the Raspberry Pi. It ceased trading in 2012 but there are still a lot of boards out in the wild, and you can still buy kits from S K Pang.
Sadly the website that told you how to use them is very out-of-date, as are the code repositories on GitHub.
I'm doing my best to fix the problem.
|Quick2Wire interface board|
I'll post information here as I update the relevant parts of the website and GitHub repositories.
GPIO Pins and the downverter cable
There's also one hardware problem but it's easy to fix.
Recent models (including the Pi 3, Pi zero and zero w) have 40 GPIO pins. Fortunately it's easy to connect them to the Interface board, and SK Pang stock a suitable downverter cable.
The Quick2Wire domain has transferred faster than expected. I'll put up a holding page later today. Resotring the current contents will take a while, but you should see some content soon.