Adding/removing channels can't find

[OttoKerner]

Discontinuation of Plugins, Watch Later, Recommended, and Cloud Sync

This morning, in our latest blog post, we announced the retirement of Watch Later (and its cousin, Recommended, for recommending Watch Later and plugin items to friends) and Cloud Sync along with the gradual phase out of plugins. While the decisions to discontinue/phase these out were not made lightly, they were done as part of our efforts to re-focus development, streamline our work flows to keep strengthening our core product, and clean house on some resource-consuming features. We encourage y…

For the time being, the only way is to do it manually:
https://support.plex.tv/articles/201187656-how-do-i-manually-install-a-plugin/