It talks of an installer, which it later transpires is only for the Mac. The instructions have since improved a bit, but initially were hopelessly confusing.
I hope the instructions will have improved even more by the time you read this, but below I’ll show you how I got it up and running.
These are instructions for a PC running Windows (7). For Mac, you can allegedly run the installer. For Linux, I expect a similar procedure will work.
Google Coder on the Raspberry Pi – How to install

Download the file

Go and download the file. It’s ~ 1 Gb and is a full system image.
https://googlecreativelab.github.io/coder/

Unzip your file

You will end up with a folder called coder_v0.4 although with time, that version number will change. Inside that folder, there’s another folder called coder_v0.4 and under that, is an SD card image file called raspi.img

Find Your disk image file

So its location will be…
wherever you unzipped the file to.../coder_v0.4/coder_v0.4/raspi.img

Flash an SD card

Flash the above .img file to an SD card (>= 4 Gb).
Instructions how to flash an SD card in Windows here

First boot

Your first boot is as a normal Raspbian boot, with screen, keyboard, ethernet. You will need to go through the normal choices screen to fully finish setting up the OS (e.g. extending the partition, etc.)
Once you reboot, your Pi should be ready to use with Google Coder.

Now Bonjour to your Windows PC

For Google Coder to work, Windows users need to install Apple’s Bonjour print services, which can be found here…
https://support.apple.com/kb/DL999
Once that’s installed, you should be good to go, as long as you have Chrome or Explorer.

Google Coder on the Raspberry Pi – How to install