Just a few comments because I had the same issue with the Logitech Radio panel in FSX just now. (Note: this is normal FSX on Windows 10, other versions and operating systems may have slightly different paths)
1. Install the appropriate Logitech plugin from https://support.logi.com/hc/en-us/articles/360024698434--Downloads-Flight-Radio-Panel
This creates the folder ?:\Program Files (x86)\Logitech\FSX Plugin with the following files (Note that there may be no SimConnect.dll) DirectOutput.dll LogiFlightSimX.exe LogiWinMediaPlayer.dll
Gauges LogiFlightSimX.xml SaiFlightFip.xml
2. Try running LogiFlightSimX.exe.
If there is no error messages, the problem might be that FSX does not actually load the program at startup.
Like suggested in comments above, open the file <User>\AppData\Roaming\Microsoft\FSX\ (e.g. with typing notepad %userprofile%\AppData\Roaming\Microsoft\FSX\exe.xml into cmd.exe) and make sure that there is an entry like this:
<Launch.Addon>
<Name>Logitech Plugin for FSX</Name>
<Disabled>False</Disabled>
<Path>C:\Program Files (x86)\Logitech\FSX Plugin\LogiFlightSimX.exe</Path>
<CommandLine>-run</CommandLine>
</Launch.Addon>
If there is not, add this entry (with the correct path) and try again.
3. If the error message is that SimConnect.dll cannot be found
3.1 Make sure SimConnect is installed, for example, by installing all three major versions, e.g., from http://www.fspassengers.com/?action=simconnect
3.2 If SimConnect is installed, it might be that Logitech's software does not look in the right place.
To find out, where the SimConnect.dll that FSX actually loads is located, you can use ProcessMonitor.exe (https://docs.microsoft.com/en-us/sysinternals/downloads/procmon)
Start procmon.exe and filter for "Process Name" "begins with" "fsx" (include) and "Path" "ends with" "Simconnect.dll". Then start FSX. Look for a line with "Operation" "LoadImage" and "Result" "SUCCESS" (this is the DLL that FSX loaded). Copy the path and copy that file to the Logitech\FSX Plugin folder.
(You can change the filter to "Process Name" "begins with" "LogiFlightSimX" and restart FSX to ensure that the logitech tool is started and finds the correct DLL).
4. A different error message appears
If there is a SimConnect DLL in the Logitech\FSX Plugin folder, it may be the wrong version (e.g., if you copied it there from somewhere). Try deleting the file and starting from 3. again.
I hope this helps.
