As far as I am aware you only have the capabilities to export mods, not import them.
This is a problem, especially if you have to develop a bigger scale mod.
If there's something I'm overlooking can you please tell me?
First go to activate mod in main menu side bar to choose the mod you want to edit, and then go into the editor. Then here you are.
You can help by EXPANDING IT.
I will also add the option to the Game Editor screen @CriticalDupe as it certainly should be there too.