Let's start by setting up an ad management plugin for WordPress MU:
Download the Advertising Manager plugin from http://wordpress.org/extend/plugins/advertising-manager/screenshots/ and install to your
mu-plugins
folder.Go to the Ads section of your admin panel and select Create New.
Paste in the ad code for the advertisement you'd like to display. (I've used a Google banner ad, which will go in the footer of the blog.)
Click Import. Advertising Manager should read the details of the ad and display an options page where you can set up some more detailed options relating to the ad.
The default options are fine for now, so just change the Google AdSense name to something shorter but still descriptive (I chose "google"), and save the ad.
To add the advertisement to your theme, open the theme file and paste the following code where you would like the advertisement to appear:
<?php advman_ad('google'); ?>
If you need to add more advertisements...