Download the latest release which is a zip file at the top of the watir package list. (At the time of this posting, the latest version is 1.0.2.) Open the User Guide for installation instructions, and check out the examples to see how you can use the tool.
For those who may not be familiar with the tool, it allows for testing web applications at the web browser level. The project utilizes testable interfaces in web browsers to facilitate test automation. Currently it only supports Internet Explorer, but plans are underway to support other browsers. It is also only available on Windows.