Twenty Eleven Theme – Adding catagories issue

Hi,
I’m getting confused trying to add a categories option to my page, which uses the Twenty Eleven theme.

I have a categories menu defined, and I can have it as the only menu there.

I also see the theme says it only supports one menu, which might explain things.

But what I wanted was to have categories of posts appearing as an EXTRA option on the existing menu, not a separate one in it’s own right… Is this possible with this theme please, and if so can some kind person point me at how to do it?

Sorry if this is basic, I am new to WordPress.

Thanks,
Nick