Method 1: Direct via Blogger Adsense gadget
Sign into www.blogger.com
Dashboard > Layout > Add a gadget > Add an AdSense gadget
Then in Layout, drag the newly added AdSense gadget to wherever you want and SAVE.
Method 2: Indirect via AdSense codes
Sign into google.com/adsense and click MY ADS at the top left, then +New ad unit, then select the ad unit you want.
After selecting the ad unit, at the bottom, click SAVE AND GET CODE
Now go to www.blogger.com
Dashboard > Layout > Add a gadget > HTML/Javascript gadget
Paste the AdSense code into the gadget
Drag the gadget to where you want and save.