Extensions: These are little apps that can be installed to work with your browser. Google Chrome is my favourite browser so I naturally download and/or use a lot of extensions that work with it. You can find loads of them in the Chrome Webstore.
So the screenshot above shows the website where you can install the extension from (yes, no need to go through the web store).
How to Install
Go to the Site
Click on the green Install button found at the top right of the site
Follow the screen prompt and
once extension is installed restart Chrome browser
Go to your video on Youtube and you will find a button under the video that says Download (as shown by arrow 1 below)
Note: You can also install the latest version of RealPlayer (google it to find the link 🙂 on your computer and it will install RealPlayer Download that pops up as shown by arrow 2 above.