I am running OpenPHT on a 2012 Mac Mini using the apple remote (or at least trying to). I switched over from PHT die to the irritation of it not keeping track of your position in a playback and viewed status. OpenPHT is fine but since moving to it I have been unable to get the apple remote to work with it. It was working fine with PHT.
Just did some checking - I switched back to PHT and the mouse works again, In openPHT pushing up and down just causes the audio sign to appear on the screen.
Did you uninstall PHT and reboot before testing OpenPHT? There’a a comment in this thread that indicates the Plex Helper background service from the old PHT might conflict with OpenPHT (this thread is about Harmony remotes, but you might find some of it useful in trying to fix your Apple Remote issue):