Actually, you shouldn't start it until the aircraft is "in the world". IE: Once you've loaded you plane into the sim, whether on the runway, ready to fly or at the gate, cold-n-dark, or whatever, THEN start FSRealWX. It can take a minute or three, depending on how many airports are nearby, like with 150nmi or so. The more airports, the longer it takes to download all the weather info, at first. After that, it keeps up with no trouble.
It receives it's location info from the sim, through FSUIPC, and keeps the weather IN the sim up-to-date through FSUIPC. It keeps up very nicely, even with a fast-mover, like an F-18 at Mach 1.05.
Once it's loaded and operating, you can get the weather at most any airport by putting the ICAO ident (like KSFO, or whatever), into the little box, center-left area. Be aware though, that every time it get's updated, it switches the local weather info (lower left area) to the closest airport. To keep the weather info for the desired airport, just hit enter again, and it will go back to displaying the current METAR info for the selected airport.
To get it to operated fully automatically, you need to go to the options window, General tab, the box labeled Startup, and make sure Download WX is checked. I have NOAA selected as a Source, and all the boxes under the Downloads heading are checked. Under the Weather tab, make sure that the Set Global WX, and Set All WX Stations boxes are checked.
Once it starts up, and begins downloading, just let it go. You will see the difference as soon as it puts the weather into the world. Once in a great while, it will lock up during the download process. Simply close it out, and start it back up, and it will function properly.
Make sure you reset your altimeter once the program is running completely
Good luck! I hope this helps...
Pat☺