With the Reality-XP software installed, there is one additional step required in order for the RealSimGear device to function with the Reality-XP gauge.
In X-Plane, locate the Reality-XP entry in the plugins menu, then under the 430/530 or 750 menu select 'Settings'.
In Prepar3D, the settings menu is access by holding the SHIFT key and right clicking near the top of the Reality-XP gauge screen.
Select the 'Use Simulator GPS Commands' option under the 'Advanced Settings' section.
In X-Plane, you can also use the Reality-XP menu to 'pop out' the GNS430/530 or GTN750 window. This will enable you to drag the screen over to the RealSimGear device display.