There are two kinds of lists on Podchaser: podcast lists and episode lists. Only podcasts can be added to podcast lists, and only episodes can be added to episode lists.
When you attempt to add an episode to a list, you'll only see episode lists as options. Likewise, when adding a podcast, only podcast lists will appear.
If you're having trouble adding a podcast or episode to a list, make sure you're trying to add it to the correct type of list.
Adding an Episode to a List of Episodes
Adding a Podcast to a List of Podcasts