How do you mean exactly? Do you have an example? The template to use for to put ads inside a message body is: ad_message_body
Then to display it only in the first post is to use a conditional statement, you can find all kind of examples here http://xenforo.com/community/resources/conditional-statements.1604/
i mean ads be in middle the post after x character , its not to be a beginning and not in finish; necessary to be in middle ....you can see image from a forum using vb
You do understand that the way of your example is not allowed by Google and can get you banned from their program? Can't help you with the correct conditional to put the advertisement after xx characters.