[SOLVED] How can I show thumbnails in excerpts

Using Wordpress 321 and Atahualpa 371

I'm trying to show thumbnails before the excerpts but I can't get it to work. I'm not sure I understand how it works. I read other posts about this but it didn't help me.

In Configure Excerpts, I have selected Only Excerpts everywhere.

In the Post Thumbnails section, I have set Thumbnails Width and Height to 150 and Crop to No. Post Thumbnail CSS is set to the default:
float: left;
border: 0;
padding: 0;
background: none;
margin: 0 10px 5px 0;

In Post Thumbnails / Post Thumbnail CSS, it says:

Where and how do I "Style the Post Thumbnail" ?

Thanks for your help
Jules