Log into your WordPress site and go to PowerPress >> Settings, Advanced Mode >> Episodes >> Check the box next to "iTunes Summary Field (Leave unchecked to use your blog post)" and click the Save Changes button at the bottom of the page. Now, you should see iTunes Summary field within the Podcast Episode box of your WordPress posts. Place the iTunes Summaries you want to use within the iTunes Summary fields of your posts and update to the posts. It may take up to 24 hours before the changes will be reflected in your iTunes Store listing.