http://www.photopost.com/manual
Now I dont think your gonna find out specific adsense info. You would need to install adsense based on their install instructions since thats there program. I can tell you how to add a header and footer in photopost and that is where people add html code.
1. ftp to your server and chmod the header and footer htm files in photopost directory to 666.
2. In Admin => Global Options you would place full server path to header and footer files filenames included on those lines and save like so
/home/mysite/public_html/photopost/header.htm
3. You can then edit the header and footer files in Admin edit header and edit footer and place whatever html you wish in those files to load adsense included.