Problems adding a sidebar to Mantra theme

Hello, I’m using the Mantra theme for a site I’m building. The site is almost finished now but I have just realised that I actually need to also have a sidebar on the left, as well as teh right.

This theme has some settings that are supposed to allow you to change the layout. However, I don’t seem to be able to changeteh setting. Does anyone know what might stop this from working, or how to get around it?

Thanks