Adsense as well as any other ad codes can be added in various positions in the blog and some positions may bring more revenue than others and it totally depends on your content and its layout.These ad codes are usually javascript codes.

The various positions where Adsense and ad codes can be added are:

Sidebar: 1. Go to Layout->Page Elements

Layout

2. Then in the sidebar you see an Add Gadget link, click on this link.

Add Gadget

3a. For Adsense add the Adsense Gadget and then configure the adsense ad.

Configuring Adsense

3b. For any other Ad code add the Html/Javascript Gadget and add your More >