1) To comment on a channel: please go to the "Channel Info" page and tap the "COMMENTS" under "Subscribed" button.

2) To comment on an episode: you could tap the the comment icon on the preview (half) page or Now Playing page. You can also comment with the timestamp, thus others could click your timestamp directly to listen to the content. The format of the timestamp is 16:51 (for example), the numbers could be changed.