How do I add widgets to center column (all posts) in atahualpa?

Hello,

I want to add a banner ad in the same spot on all posts. I've added the banner code to a new widget but can only place the widget in the loop, which isn't where I want it to appear. I'd like it to be embedded in the same place in each post. Any help with this will be much appreciated. (I managed to get a banner ad to work in the header of my website--no problem with the side bar either.)

Thanks,
Chloe