If that .REG file is for KLAS, if you double-clicked on it and it confirmed the information was written in the registry, the first thing it does when you launch FSX, is that will ask if you want to activate it, and it will keep asking at every launch, until you activate it.
So, either you haven't double-clicked on the .REG file, or that .REG file wasn't for KLAS but, even if it was for something else, you should at least see a request to activate it.