To add a video onto your site, the video needs to be uploaded onto a service like YouTube or Vimeo. Once you've uploaded it, you will need to find the embedding option. This will give you a piece of code that you need to copy and save for later.
To add a video in Version 5, navigate to the page you wish to add a video to and open the editor. See here for more details on how.
Find where you want to place the video within the content and click there. Then click the third icon from the left (looks like a piece of film). A new window will pop up. This is where you need to paste the embed code you will have copied earlier.
Once the code is in, press Save & Close, then Save & Close again to save the entire page. You now have a video on your page!