Look for showaddelement='no' and replace with showaddelement='yes',then save the template.Click the "Page Elements" and observe that an additional "Add a page element" have been added to the top of blog post.
If you want add adsense script or HTML/javascript above the main column,you just click the link "Add a Page element" and select the "HTML/javascript" from the pop-up window,Click "add to blog",past the code,save the widget.