Also make sure you're launching "StardewModdingAPI.exe" and not "Stardew Valley.exe". You can switch between vanilla or modded by choosing the correct executable to launch.
I have tried many times to download Stardew Valley Expanded mod, only for me to either be unable to use it or, if I try to use a mod application, it won't register the game at all despite the game being on my taskbar and open. There is even a file for it and it still doesn't register. Is there another way to get it?