You have two choices available either installing the CSS Friendly Adapters source or the compiled .dll. Selecting both "Download Source" or "Download" under the Getting Started paragraph both will take you over to CodePlex.
When you get to CodePlex click the Releases Tab and select either the source zip file, unzip it and follow the instructions in the Readme file or download the CSSFriendly.dll and CSSFriendlyAdapters.browser file. If you opt for the second route just add a reference to the CSSFriendly.dll and add a App_Browsers folder to you project or website and place the CSSFriendlyAdapters.browser file in it.