Their directory uses a two-level categorization system. The categorization seems pretty effective.
Each podcast has its own page, and you can listen to the podcasts without leaving the web site. They also provide assistance to subscribing in typical podcast aggregator applications (iTunes).
It's totally a mystery about how to add a podcast to their directory. There is no "Suggest" link anywhere. I would recommend using their "Contact Us" page and select one of those addresses.