thanks for asking.

Remote Buddy sends actions as UDP packets to Plex. If UDP traffic is blocked, or port 9777 of your Mac is already in use by another application (so Plex can't use it), Plex won't be able to receive the actions.

Here are a few questions that may help in narrowing down the problem:

1) Can you control Plex with your Apple Remote when Remote Buddy is not running?

2) Are you using Little Snitch or any other firewall software?

3) When Plex is running, please enter this line in Terminal.app and hit return:

sudo lsof | grep Plex | grep UDP

Does the command produce any output? If yes, please copy and paste it in the reply.

Thanks in advance.

Best regards, 
Felix Schwarz