Well I copied all files to the rFactor data directory (same where the "Launch rFactor.EXE" is located). Still not working and meanwhile I figured out that the process seems to cause an exception.
https://lh4.googleusercontent.com/-G44gMG6BQ4A/UV8ZgmmS0_I/AAAAAAAAAC0/yo3exkV0nwU/s800/Fehlermeldung_rFactor2WorldWeather%2520Tool.jpg
P.S.: If I move the files to a different plugin there is at least a message saying that the xml file does not reference a rFactor2 data directory - thus the directory should be o.k.
Is there some specific information that needs to be in the ./UserData/Config.ini file? Or do I have to have certain mods or components installed (currently majority is F1 mods).
After deleting all locations of the XML file - except one where I was confident that I already did same laps - I was finally able to start the tool as well.
Conclusion: Tool is currently very sensitive to syntax errors in the XML file or if locations don't exist in the expected directory.
Nevertheless great idea for a PlugIn