The "mods" folder in your Starbound installation directory. To access it, go to Steam, right-click on Starbound (in your game library), choose "properties", then "local files" and click on "browse local files". There's a cool .gif in this tutorial if you still don't find. You can also find it if you search a bit in your steam installation: for instance, on mac os it's in ~/Library/Application\ Support/Steam/SteamApps/common/Starbound/, on windows it will probably be something like C:\"Program Files"\Steam\SteamApps\common\Starbound\ Edit: if there isn't any, you can create it (as long as you're in the Starbound directory)
