Case 1: The mod folder comes with a "GameData" folder
As pointed out by @MaltYebisu, this method doesn't work on MacOS. Mac users, please scroll down a bit and read about case 2
If the mod folder contains it's own "GameData" folder, you can drag and drop it directly into your game directory, which also contains a "GameData" folder. This will merge both folders together (should work on each operating system):