I use Atahualpa 3.7.1. I am approved by Google Adsense, which supplied the following code:

<script type="text/javascript"><!--
google_ad_client = "ca-pub-3103819256446759";
/* Left Sidebar */
google_ad_slot = "9827483533";
google_ad_width = 120;
google_ad_height = 600;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>

I installed the plugin Ad Injection Version 1.2.0.7, dragged and dropped it into the left sidebar but nothing appears on the website: www.windycityonthecheap.com. I did the same with the plugin WYSIWYG Widgets. Again, nothing.

Am I correct that I need to insert the code supplied by Google Adsense into a widget? Or, am I doing the wrong thing?

I don't know about that plugin but I just put that code in the text widget in the sidebar and it worked fine.

Just for the fun of it I installed the Ad Injection plugin and that worked fine as well.

I am having the same problem! I have tried repeatedly to insert my adsense code into the sidebar text widget, the space shows but not the ads. I checked the code by inserting on a blank page and uploading, the ads appear.

I found the solution in a previous forum post - working now!

setting the CSS: Compress? to NO in ATO>Configure CSS & JS

Glad it worked for you. I also use Ad Injection 1.2.0.7 with Atahualpa 3.7.3, but it worked (inserts adsense code into the sidebar text widget) with Atahualpa's default option CSS: Compress? = YES in ATO>Configure CSS & JS. See the results at goo.gl/CKiLg.