MyBB Community Forums

Quote:Install this plugin: http://community.mybb.com/thread-31860.html

And then add the following code BEFORE ALL code (if you want to show "before" first post) of your theme's postbit and postbit_classic templates OR paste that code after {$post['message']} code (if you want to show "within" first post);

<if ($postcounter - 1) % $mybb->settings['postsperpage'] == "0">
// Show ad code here.
</if> 

Getting some error while adding. Thread Message is not showing

what is the error you are getting and which plugin you have actually added ?
in general, template conditionals plugin should be sufficient (no need of php in templates)