You should be able to use either the theme editor to edit the part where you want to put the adsense. Alternatively you can also add a text widget and drop te javascript in there.
As I said, all themes are different, and use different files, sidebars, and widget structures. If I had the site page in front of me, I can provide a guide saying which code to Find, and add below etc.
Just modify your header with the adsense code or you can create a text widget and place your adsense code. That's all!!!
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Ive asked you for a link.. link me to your site and I will supply you with a small tutorial... PS - Don't double post.