Content can be added below the left sidebar in several ways. If you only want it to be visible on a single page, use the Left Sidebar Content Editable Region on that page.
To display content on multiple pages, the content will be added to a _props.pcf file. Remember, any settings made in a _props.pcf file will carry through to all files and subfolders within that folder. Add it to the file in the root folder to carry through to the whole site. This field is available on all _props.pcf files.