Add a breakpoint

How do you add a breakpoint in BSS?

I would assume you are referring to the Media Queries...

Double click on your custom CSS file in the Design pane (bottom/right). Click on the triple stacked dots of the CSS rule you want to have a media query applied to and select "Add Media Query". This will create a media query based on the device size you have currently selected in the Device Toolbar next to the Options pane (top/right). You can click on the media query text to edit it like you would with a CSS rule or property.