I bought and installed FSRealistic today and it started out great. I assigned First Person to a key and it worked the first couples times.
Then, when I noticed nothing happened when I pressed the key, I opened the app to see that the key binding option is missing from Settings.
If I open Disabled effects, First Person is unchecked like everything else, yet it's missing from the settings tab and my key bind does nothing. I've tried restarting the app and game.
Could you please share with us a screenshot of your FSR window? Please take the screenshot while you are inside MSFS and a flight is started (good indication is the you see the effect tab)