
I have just taken a random skin from ManiaPark TM Car skins, more precisely this nice Audi R8C.
I Downloaded the skin zip and the locator file, placed them in my ManiaPlanet documents (Skins\Vehicles\CanyonCar),
started the game, then the Canyon station, and then "Editors" and finally "Paint a car", where I selected the Audi.
Result here:

Attention

Now, if you don't have access to the editors and therefore also do not have the car painter, then you have the demo version, which gives only access to multiplayer gaming and custom titles, but you cannot use custom skins.
You need to pay for the game to be able to use single player gaming and custom car skins.