In order to get a flight plan into the X-Plane 11 GPS (GNS530), the following procedure should be followed.
Create a flight plan. The program used can be your choice; I use Plan-G for this but you may use a different program. Save it as a plg file (if using Plan-G) and then, using the 'Export' button, export that same flight plan to your X-Plane 11 installation directory. My X-Plane 11 directory is C:\X-Plane 11\Output\FMS plans. The flight plan will be saved as an fms file.

Now run X-Plane 11, and after selecting your aircraft, departure airfield, time and date hit 'Start Flight'. In order to preserve your aircraft's battery, it is best if you start the aircraft engine prior to doing the following.

For this exercise I shall be using the C172SP. So, having started the engine, click on the centre of the GNS530 panel to bring up the pop-up display of the GNS530. (it just makes things easier).

Hover the mouse – a little hand – over the 'CLR' button and press for about half a second. Nothing will appear to happen. Release the mouse button, then move the cursor, again a little hand to the 'FPL' button and press; this should bring up the current, or previous, flight plan. Now move the cursor to the bottom right button marked 'PUSH CRSR'. A little hand should appear. Do not press the cursor, but move it slowly to the right until it changes into a small curved arrow. Using the scroll wheel on the mouse button, scroll away from you just the one click; this should bring up the flight plans in your X-Plane 11\Output\FMS plans folder (Flight Plan Catalog). Now move the cursor back to the centre of the 'PUSH CRSR' knob until the cursor changes into a little hand and press just once. The first plan should now be highlighted. Now move the cursor to the right again until it changes into a large curved arrow. Scroll the mouse wheel away from you to highlight the flight plans lower down the list, or towards you to highlight the plans higher up in the list. When the required flight plan is selected, press the 'ENT' button above the 'PUSH CRSR' knob.Your selected flight plan is now the active flight plan and is showing in the GNS530 window.

Below are the links to the manuals for the G530 and G430 from the X-Plane site.

X-PLane 11 G530 manual
X-PLane 11 G430 manual