Before you do the mod, please ensure you have make your own back up. This trick only tested at Babylon Theme only, for other theme please make necessary changes.
File to adjust : BoardIndexTemplate.php
Find this in the file,
function template_main()
{
global $context, $settings, $options, $txt, $scripturl, $modSettings;// Show some statistics next to the link tree if SP1 info is off.
echo ‘
Add the following code below the above code
<center>ADD YOUR ADSENSE CODE HERE</center>
Dont forget to add your adsense ID.
Screenshot
Popularity: 1% [?]
Thanks