Due to Google politics you cannot install extensions to your Google Chrome web browser directly from website. (picture 1) Therefore you need first to download the extension (picture 2) and install it manually.
To download the extension click the Download button and then Continue (picture 2). Once you download it, in your Google Chrome addres bar type chrome://extensions and this will lead you to Google Chrome extensions manager page (picture 3).
Locate the downloaded extension (ChromeSiteSearch_latest.crx) it should be in your Downloads folder by default, or your desktop (for example), and drag and drop the extension in Google Chrome extensions manager page (picture 4).
Finally, confirm the installation by pressing Add (picture 5) and ChromeSiteSearch extension will be installed on your Google Chrome web browser.