Full documentation is still under development at

http://podcasterswiki.com/index.php?title=PodPress_Documentation#Premium_Podcasting

This is the short of it is that you need to get and install the

Role Manager plugin
http://redalt.com/wiki/Role+Manager
Anyone that should have access to the premium podcasting files 
need to have the Premium Content role, which can be done by
making them Premium Subscribers

Then just in each post set the media file as premium content 
and normal visitors will not be able to see the content 
via the web or from the feed.
Put premiumcast.php in your main wordpress dir
Then have your subscribers use this file as their rss feed.
It will ask for their user/pass before giving the RSS feed.
Juice and iTunes supports this fine.

Keep in mine, that this does NOT protect your content if someone 
discovers the URLS, it only hides the location from showing up
on the site or in the feed. To fully protect your files I have 
also been able to get this working with aMemberPro
http://www.amember.com/
aMemberPro will protect the files from being downloaded at all, 
unless authorized. It also handles monthly subscription issues thru
paypal and such. Its a great tool, and combines with WordPress and podPress
you can have a full blown premium content podcasting service.