Step 1: Login to your blog as administrator
Step 2: Click on Appearance
Step 3: Select Widgets
Step 4: Select the sidebar you want (Default Sidebar 1)
Step 5: Add Text widget
Step 6: Click on Edit widget
Step 7: Enter Caption. I have used “Useful Links”
Step 8: Paste your Google Adsense code in the text area. See example code & screenshot

Example (Copy and Paste below into Widget):

<script type="text/javascript">
<!--
google_ad_client = "pub-9990819195828769";
google_ad_slot = "7810166087";
google_ad_width = 200;
google_ad_height = 200;
// --></script>
<script src="http://pagead2.googlesyndication.com/pagead/show_ads.js"
type="text/javascript"></script>

Screenshot

Wordpress Text Widget - Add Google Adsense code

Step 9: Click done & Save changes.

You should now be able to see Ads from Google showing on the left side of your sidebar. Just like we have on the top right of this page.