Once you have an approved AdSense account, there are 2 ways to add AdSense ads in your blog
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
Paste the AdSense code into the gadget
Drag the gadget to where you want and save.