Once you have created or download a custom content it's normal to use it, to do it when you have the file (usually a "*.zip"), you have to place it on the following folder depending of the nature and the environment of the content:
By default
[User Data]isC:\Users\***USERNAME***\Documents\ManiaPlanetyet you can specify the location yourself when you install Maniaplanet or via the Maniaplanet Launcher (with theHelpbutton).
For skins & 3D models:
[User Data]/Skins/Models/CanyonCar/[User Data]/Skins/Models/ValleyCar/[User Data]/Skins/Models/LagoonCar/[User Data]/Skins/Models/StadiumCar/[User Data]/Skins/Models/ArenaPlayer/For mods:
[User Data]/Skins/Canyon/Mod/[User Data]/Skins/Valley/Mod/[User Data]/Skins/Lagoon/Mod/[User Data]/Skins/Stadium/Mod/[User Data]/Skins/Storm/Mod/For signs: [User Data]/Skins/Any/Advertisement/
For editor plugins: [User Data]/Scripts/Editorplugins/