Paste Adsense code but it disappears.

I have been trying to paste my adsense code into Source on the dropdown menu of my site. I copy, paste, hit save, exit, go back to my site and nothing happens.

I am so frustrated. I am trying to get the Adsense Leaderboard to appear either above or below the header.

What am I doing wrong?

To clarify…the leaderboard is the style of the ad I wanted on my site.