if you are loking to add coe to your head section then why not try my training Google verification in four easy steps and instead of the google code copy the adsense code.
if you are after placing the ads then you can add a text box to your sidebar and copy the ad there.
good luck phil

Morning
Did you activate your site with google?
Then best way is to paste the code in your all in one seo plugin under general sertings

Hope this helps a little