Google Pages does not allow you to currently place Google AdSense ads on your pages. A WebmasterWorld thread has a reply from AdSenseAdvisor stating that AdSense is currently not supported on Google Pages. AdSenseAdvisor does say that Google hopes "to offer this feature soon (although we don't currently have a date for when this option may become available)."
In the meantime, they suggest you set up a blogger.com site and host Google AdSense on it. :) I guess it is better than hosting trojan horses on Google Pages. No comment.
Forum discussion at WebmasterWorld.