Click the "insert/edit media" button in the content editor to open the media toolbox.

If the video is in your webfiles simply browse and select it. If it's on YouTube or Vimeo you can use their "sharing" option to get the embed code and paste that into the box under "embed". 

You can specify the width and height (in pixels) to set a fixed size for the video or if you'd like the video to be responsive and automatically scale up/down with it's content container you should begin by inserting a Responsive Mediaideo Container and then insert the video inside that box. ("Insert Layout Blocks" > "Grids and Containers" > "Responsive Media Container")