You will be redirected to an external website to complete the download.
The Download link takes you to the root directory of all available Chromium OS builds. You need to select your operating system and build number in order to download the correspondent files. Chromium OS is only available in source code form. For advice on how to compile the source, see this discussion on the developer's forum page. A disk image that works on Intel Macs created by one Chromium OS user is also provided here when you click download.