To do this, I did not set up a behaviour, I simply Paired the Apple Remote with RB and then set Global commands to Activate Frontrow, etc...

I did this to rule out issues with PLEX. 
I will try to reinstall PLEX tonight, just incase the About PLEX is lying. If this doesn't work, I'll completely delete the PLEX behaviour I created and try to set up a behaviour for Slingplayer..to see what this does.

Again, I don't think it's an issue with RB or PLEX, I think it might be something I'm doing wrong or missing.