New to podcasting and I'm stuck... please help

Question

I used the application Feeder (from reinventedsoftware.com) to create my XML file and publish it to my server. I successfully submitted the podcast to the iTunes Music store, but when I try to subscribe to my feed, I receive the following error message: Does not seem to be a valid Podcast URL.
I tried validating my feed at feedvalidator.org and it did not validate, but I simply do not understand the two error messages it listed. I wasn't able to troubleshoot the problem with the podcasting technical spec either.
Here is the link to the feed:http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewPodcast?id=120926616&s=143 441
17" PB Mac OS X (10.4.4)

Answer 1

1. Do I simply add a new <item> section to my
existing XML feed? (As opposed to replacing the one
already there.)
Yes - you add in a new Item section for each new show.
2. If so, do I add that new section above or below
the initial <item> section?
It is best to add it above the most recent one. The one exception. If you have a promo - make it the first physical item in your feed. This way when someone does a search in iTunes - if they click on you podcast in the list of podcasts - it will play your promo first.
3. I'm archiving sermons, so at the end of the year,
will my XML feed basically have 52 <item> sections in
it?
Yes, correct.
http://www.podcast411.com/howto_1.html
Here is a detailed tutorial on Creating an RSS feed and what should go in it.
Rob @ podCast411