Use:Advertisement Management
Code: 
http://www.phpbb.com/customise/db/mod/advertisement_management/
Allows you to:

  • Create new advertisement positions for on the forum (and even off the forum).
  • Full HTML for advertisements (Google Adsense will work).
  • Count the number of times an advertisement is viewed and how many times it is clicked.
  • Set limits to the number of views, clicks, or time before the advertisement will be disabled automatically
  • Set a priority to let you decide how often an advertisement should be displayed compared to others in the same position.
  • Disable specific groups from seeing advertisements
  • Select specific forums to display advertisements in