Sorry to clarify you select that element in the HTML section of the toolbar as so: http://snag.gy/ng33R.jpg (outlined in red)
Then look on the right-hand side for the media query: http://snag.gy/cpHY6.jpg (outlined in red)
When you resize the screen it should automatically update the CSS on the right-hand side with the relevant media query, e.g: http://snag.gy/CHNKa.jpg