Where do Kerbal Space Program add-ons have to be placed (with the Steam version)?

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I use the Steam version of Kerbal Space Program and I don't know where add-ons have to be installed.
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Where do I put mods for KSP on steam?
If you've downloaded the game on Steam and can't find the file folder, right-click the game in your Steam library, select Properties > Local Files > Browse. In your KSP folder, open the GameData folder. Drag the mod file folder to the GameData folder. That's all there is to it.Where does Mechjeb install to?
Locate the KSP folder and click on GameData. You should see a "Squad" folder if you haven't modded your KSP version yet. Now drag your 'MechJeb2' folder into the game folder and tada! You have now successfully installed an addon.How to install KSP mods for steam
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