[twenty twelve] center a custom widget?

I made a custom area for new widgets on my website’s front page but now I am having trouble centre aligning one of the widgets.

My site

I want to put the archives section in the middle. I have used firebug to find the widget but can’t see it anywhere (I am a n00b, forgive me).

Does anyone have any experience with this? Any help greatly appreciated!

Thank you!