How to put Adsense Leaderboard under custom header

I have a custom header on my blog http://table4five.net. I’d like to put an Adsense Leaderboard, which is 728×90, underneath the button that scrolls back through my previous posts. I’ve tried pasting the Adsense code into header.php which didn’t work, and at the top of the main index template, which also didn’t put it in the right place.

Into which file would I paste the Adsense code so it stays at the top of the post area under the header?