Launch Terminal from your Applications:Utilities folder.
SSH into the Apple TV by entering the following command into the Terminal window. Replace "10.0.1.20" with the IP address of your Apple TV.
If you haven't connected to this device before it will ask you if you're sure you want to continue connecting. Type in yes and hit the enter key.
Enter alpine as your password if you haven't already changed it.
Add the new Cydia sources by inputing the following commands into the Terminal window.
echo "deb http://apt.awkwardtv.org ./" > /etc/apt/sources.list.d/awkwardtv.list
echo "deb http://www.ambertation.de ./downloads/PLEX/" > /etc/apt/sources.list.d/plex.list
Now update your Cydia database by entering the following command.
Install the plugins (and its dependencies like beigelist by DHowett) by entering the following command.
apt-get install plex-client-plugin
If you are warned that additional disk space will be used, input Y and press enter
If asked to Install these packages without verification, input Y and press enter.
Restart the GUI by entering the following command into the Terminal window.
Once the GUI restarts you should see PLEX in the menu bar.
You can learn more about these instructions from their source.