I just logged into my own (unmonetized) Blogger account and found out the exact steps: Login > Layout > Add a Gadget > AdSense Not exactly brain surgery.
Its so simple a child can do it just go to layouts add a gadget look for adsense pick three 1 for below the header and 1 for the right column and 1 for the footer. If you do not have adsense account get one first. Now your done good luck