Every section of a Remixer page has a 'New' button in the top center of each 'Web Section'.
Clicking on this button reveals a list of all built-in 'Web Sections':
When you've located the 'Web Section' you wish to add, click it. It's automatically added to the page. You can then move that section up or down the page by editing the page structure.
It's currently not possible to rename a 'Web Section'.
Find a specific type of 'Web Section'
When you click the 'New' button you can then specify which type of 'Web Section' you're looking for. For example, if you want to add a video, click the 'Video' link.
A list of 'Web Sections' that offer this feature is listed. In this example a single 'Web Section' shows since only one 'Web Section' is available for video.