The IR sensor is in the plasma panel not the media center.
Are there any other Pioneer components in your system?
If there are flash them and use their "Control Out" to "Control In" on the media center. Control In disables that components' IR sensor so you want be able to control the plasma panel by aiming a remote at it but I gather that doesn't concern you in your system. Pioneer's Control In/Out ports are for Pioneer control signals only (called SR for System Remote).