I have a Raspberry Pi 3 model B with a SanDisk ultra 32 gb SD card. I used etcher to flash the SD card with the DexterOS-1.0.3-Final.img file that I downloaded here, from your link. Now when I boot the pi, I am prompted with:
Thank you for downloading DexterOS. This OS is not meant for people who want to log in but is meant for people who just want an easy to use environment to learn programming. It is meant to serve a headless GoPiGo (without a monitor) and as such, doesn’t require a login.
You can learn about DexterOS here