You can search for mods in SFS forum.

Note: the folder should have the same name as mod entrypoint dll. For example: if I have a mod named Test.dll, my folder should be named Test

1. Go to "Spaceflight Simulator_Data/MODS" folder
   

2. Now you only need to drop mod folder inside like this
   

3. Execute the game

Note: the folder should have the same name as mod entrypoint dll. For example: if I have a mod named Test.dll, my folder should be named Test

1. Go to "Spaceflight Simulator" Folder
   

2. Right click on "SpaceflightSimulatorGame.app" and click "Show package content"
   

3. Go to "Contents/MODS" and drop mod folder there.
   

4. Now execute the game