The OP specifically asked for how to access the video on demand solution in his browser.
Back when this was posted, the version of Plex Web installed on a fully updated server did not (yet) support this feature.
Should now be included in the latest beta of the regular Plex Media Server as well (as of version 1.18.6)
As for the desktop players… as stated in the related support article and blog entry:
Plex is working to bring this feature to other apps soon.
