John the file you downloaded is a .zip file which is a type of file compression the packs a lot of data into a smaller file. Once you download the .zip file if you're using Windows right click on the .zip file that you want to unzip (uncompress), and select "Extract All" in the context menu, if it ask you where to extract the data folder to just choose a location that you can find easy like your Desktop, My Documents or Downloads folder. Once you extract the folder double-click on the folder and when it opens select the "index.html" file and it will open up in your default browser.
The second option is to just double-click the .zip file you downloaded, depending on your system it should just open up a file browser with all the files inside. Then just select the index.html file and it will open up in your default browser.