I understand your frustration with the RFN gauge in FSX Steam Edition not functioning properly on your Windows 10 machine, and I'm more than happy to help you out. Since you have successfully installed it on an older computer with Windows 7, it's evident that the issue is specific to Windows 10. Let's address this step by step.
Compatibility mode: Firstly, you may want to try running FSX in compatibility mode for Windows 7. To do this, locate the FSX.exe file (usually found in C:\Program Files (x86)\Steam\steamapps\common\FSX), right-click on it, and choose 'Properties'. Navigate to the 'Compatibility' tab, tick the 'Run this program in compatibility mode for:' box, and select 'Windows 7' from the drop-down menu. Click 'Apply' and then 'OK'. Launch the sim and check if the issue is resolved.
SimConnect compatibility: The RFN gauge relies on SimConnect to function correctly, and FSX: SE uses a newer version of SimConnect than the boxed edition. However, FSX: SE also includes the legacy SimConnect libraries for backward compatibility. To ensure that the RFN gauge is using the correct SimConnect version, navigate to the FSX: SE installation folder, and then to the 'SDK\Core Utilities Kit\SimConnect SDK\LegacyInterfaces' folder. Here, you'll find three folders: FSX-RTM, FSX-SP1, and FSX-XPACK. Install the SimConnect.msi file in each folder, starting with FSX-RTM and then progressing to FSX-SP1 and FSX-XPACK.
Check the panel.cfg file: Verify that the panel.cfg file for the aircraft using the RFN gauge has the correct entries. The lines should look something like this:
gaugeXX=RFN_Gauge!RFN_Gauge, x, y, width, height
Replace 'XX' with the next available gauge number, and 'x', 'y', 'width', and 'height' with the appropriate values for the desired position and size of the gauge on the panel.
Reinstall the RFN gauge: If the above steps do not resolve the issue, you may want to uninstall the RFN gauge, download the latest version again, and perform a fresh installation. Make sure you follow the installation instructions provided with the gauge.
I hope these suggestions help you resolve the issue with the RFN gauge in FSX Steam Edition on your