If the Plex remote isn't working then you may have to make sure Plex is configured to allow remote control. Also make sure you are using Plex Server and Plex Home Theater.
Make sure you you have installed Plex Server and Plex Home Theater. Start Plex Home Theather on your computer.
Select the "settings" icon in the left most menu by using the left key. Then select "Services" in the middle menu.
Make sure the settings "Allow control of Plex/HT via HTTP" and "Allow control of Plex/HT from Plex Clients" are selected.