I have plex on my Windows machine (Server and Player) then the win 8.1 app.
The Raspberry Pi is already working and playing most of the things I opened on it (With a keyboard connected).
Now what I want is to control it via my win 8.1 tablet.
Every app is connected to my account.
I can control my computer, but it doesn't find the running RasPlex.
If I try to get it started of the plex.tv --> Launch box the RasPlex shows up but doesn't support the playback of any media.
Do you mean newest stable release (from October last year) or the newest experimental build RP-9.9.19 (released four days ago) ?
Casting to RasPlex from Plex/Web will probably not work with the stable release, but it does work for me using RP-9.9.19.
(It also worked with some of the earlier experimental builds.)
If you are using an experimental build and it isn't working for you, that could be because you didn't configure the myPlex login properly in the "Preferences for System/myPlex" submenu, or that you 'unticked' one of the flags in the "Preferences for System/Services" submenu which allow PHT to be controlled over network.
But most likely your main problem is that you are trying to use the new Plex/Web beta version at "plex.tv/web/app" which does not yet have any casting ability.
You will need to use the old Plex/Web version built into your PMS server instead. This can be done by browsing to the server's address with the addition of ":32400/web" to the URL or IP address of the server machine.
eg:
In my LAN my PMS server has the IP address 192.168.10.101, so from another computer in this LAN I can reach its Plex/Web client scripts by entering the following line into the address field of Firefox: "http://192.168.10.101:32400/web"
If you need to access it from a computer not on the same LAN, then it is the WAN IP or URL of your LAN that must be entered into the address field of the browser, still with the addendum ":32400/web", and your router will need to be configured to pass such http requests to the PMS server machine.
