To add a video to the description of your product you first go to Catalog -> Products and click on the blue button at the product you want to add the video to. You will be directed to the following page.
As shown in the image a video can be added by clicking on the video image in the menu of the product description. After clicking on this button the following page will show up.
Copy the url of your (Youtube) video and paste it in this block. When this is done the settings page will look like this
Following these steps will make sure to add a video to your product description
Don’t forget to save the new settings/changes.