Go to http://extensions.joomla.org/ and do a search for Adsense. There are quite a few adsense extensions/plugins to make inserting Adsense much easier into Joomla.
Check these links out: http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,243/Itemid,35/ http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,25/Itemid,35/ http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,26/Itemid,35/ http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,890/Itemid,35/
you can use http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,1702/Itemid,35/ mambot which is developed by me
i personally use this one: http://www.joomlaspan.com/joomla-downloads/Download/-JoomlaspanModules/GoogleAdSenseRelatedModules/ which is real easy to install and use... you can have your adsense code up and running in less than 5 minutes.