How to add Google AdSense without plugin in first post of thread
I am using MyBB 1.6.8, no any kind of plugin installed using default MyBB theme.
You'll need a plugin with this since there isn't (or to my knowledge) a template that only shows the first post in a topic (there are only templates that are for the whole posting area, topic view, etc.)
If you mean after, does this work?
ACP > Templates and Style > Templates > Default Templates > Show Thread Templates > Showthread
Find:
<div id="posts">
{$first_post}{$posts}
</div>
(Updated 13/04/13. If you do not have this in your templates, I am using xThreads.)
Add your AdSense code between {$first_post} and {$posts}.
If not, sorry, but I also don't know of such a template.