Here's the exact steps to set the explicit tag:
1.) Log into your WordPress site
2.) Hover over the PowerPress link in the lefthand sidebar and select Category Podcasting
3.) Click the Edit link underneath the One Thing Audio category
4.) Go to iTunes tab, make the correct selection from the iTunes Explicit dropdown menu, click the Save Changes button at the bottom of the page
If you've done all of these steps correctly, and you have a caching plugin turned on, deactivate that plugin temporarily, as it may be "holding on" to an older version of your feed.