Add PHP code in Post/Page Info

Hello,

I using Atahualpa theme since years ago and always running ok, but now I wuold like to have extra power of it.

In my home page I have publish a banner with a 468 x x60px. When somebody push the title of the news and go there I have the possibility to add a banner of 728 x 90 px. I trying to add some php code in Post/Page Info but is not running.

How I can add PHP code in the Post & Pages?

Thanks a lot,
Alex