One short general note: 
One should avoid to create a new Behaviour for an application that's already supported by Remote Buddy, because it really is an unnecessary waste of time spent on duplicating functionality that's already ready to use. Instead - if an action you need is missing - the recommended way is to extend the existing Behaviour via "Custom actions" (=> available in the action selection popup at Prefs > Mapoing > [Behaviour of your choice]).

What is likely happening in your case is that your Behaviour (and the mapping you defined for it) never gets used because there already is a Plex Behaviour and it takes priority. The way around this is to deactivate the "factory supplied" Behaviour by unchecking it at Prefs > Behaviours, so that your home-made Plex Behaviour is the only one that remains.

I'd also like to recommend you to take a look at this blog entry for an easier, faster (takes maybe one or two minutes) and proven way of using Plex' Harmony Remote Mode (including the Plex Harmony profile) and using the Harmony Remote as an Apple Remote replacement in all other applications:

http://www.iospirit.com/blog/article/131/Remote-Buddy-and-Boxee-Plex-and-XBMC/

Best regards, 
Felix Schwarz