Go to Admin CP > Style Manager > Edit Templates > Ad Location Templates > Ad_Showthread_Firstpost Insert google adsense code there and it will show in the first post of every post.
No. It'll display an advertisment AFTER the first post. Not inside the first post. To the OP, if you're still searching for how to add google adsense inside a post, Google this vbulletin hack: Elmati ads inside postbit
Using templates is WAY more powerful! With the feature you are limited to... No Google AdSense Ads Low - one small text-only advertising unit under the navbar and one small unit in the footer High - one medium advertisting unit under the navbar, one unit before the footer, and one unit for guests in the first post of a thread With templates you can put them nearly anyplace! In fact, if you follow the advice of Site Reviver you get MANY MANY options already (semi_ built in) to vB. check out this thread for more information on vBulletin Ad Integration. Which offers the following locations: ad_footer_end ad_footer_start ad_forumdisplay_afterthreads ad_forumhome_afterforums ad_header_end ad_header_logo ad_navbar_below ad_showthread_beforeqr ad_showthread_firstpost And even more if you have the blog installed!