Plex integration with Multistream operates at two levels. You have the usual playing of a URL, or you can connect directly to your Plex server to get access to searching and browsing of your own content!
If you use Plex without configuring Multistream, you have to post the full URL to the track. This is useful for playing your Plex served content on other peoples Multistream devices.
Go to Plex and locate the video or music you wish to play from your library. Look for Get Info menu item.
Copy the View XML link for the stream to your computers clipboard.
Make a request from your StreamIT.ME device and supply the URL obtained in Step 2 by pasting it in.
By doing this, you will be able to search and browser your Plex servers content from within the Multistream user interface.
From your in-world receiver, access the Web Control Panel and follow the instructions.
Alternatively, you can go directly to StreamIT.ME and login to your account. From there, go to Your Account -> Receivers, locate the receiver you wish to associate with your Plex server and click the link to edit it.
Enter the details for you Plex account. You will only have to do this once.
If successfully authenticated with Plex, you will next be presented with a list of your Plex servers. Choose the one you wish to associate with.
That's it. You can now use the Search and Directory features inside Multistream to play content from your Plex server.