It (should) show up as the main action (where play/watch now is) if you’ve got Plex Sync set up and it’s syncing items into your Trakt Library (previously Collection) so that we know it’s “in your plex library” and therefore can offer that option as the main action.
I don’t use plex sync myself and this is the only info I have right now; Plex Sync BETA, so I’ll have to see if someone else can point you to how that works, if necessary.