Hover over the PowerPress link in the sidebar of your WordPress dashboard. Hopefully, a link will show in the popup menu for Podcast Channels. If it's there, click it. Hopefully, you'll see a listing there for your podcast. If you do, click the Edit link under the name of the podcast. Click the iTunes tab. There, you should see an option for the explicit tag. Choose one of the options in the dropdown menu and click the Save Changes button.