ActiveForums is the awesome forum module used on this site. One of the great and under-used features of this mosule is it's templating capability. In this tutorial I will show you how to update the templates so that you can include advertising like you see on this site.
The first thing you will need is an adsense account and to run through the AdSense setup wizard.
If you don't already have an Adsense account you can get one now:
Now use the Adsense Setup to get the Ad code.

Now what you need to do is login with admin privileges and navigate to the control panel for the active forums module.

Assuming you already have your groups and forums setup you can enter the templates area of the control panel.

Now we need to select a template to modify. I'll start with the ForumView, which is the intial list of all the Groups and their respective Forums. To make your ads show up right below the breadcrumb and just before the first group locate those tokens in the ForumView Template and paste in your adsense ad code. I also put a paragraph element around the code to align it.

After pasting in the code to the Forum View template like shown above and clicking on update you may need to go into the Main options and make sure that the ForumView Template is actually the one that will be used.

Note: You may need to restart the application to actually have the new template get picked up. Before doing that, you may also want to put ad code into one or more of the other templates.
The image below shows the code needed to make the Google Ads appear after the first post in a thread. To find this location in the template you can looking for the closing /Topic tag. In this example I am actually adding a new row < tr > to the table that holds the first post (the Topic), and putting some styling on it so that it blends in with the replies.

This image below shows the location of where the Ads will display when modifying the TopicView template like in the image above.

And that is it. Please reply with any feedback or questions.