You need to pass it an RSS feed from a podcast.
Here's an example. Let's say you like listening to CNET's Buzz out Loud.
Go to the podcast's website:
http://bol.cnet.com
There's a link on the right side "Subscribe to this podcast using an RSS reader other than iTunes"
That's the URL you want to type into MobileCast:
http://www.cnet.com/i/pod/cnet_buzz.xml
Now you can look through the episodes inside the app and download the ones you want.