locate your installation folder of stk
put kart_characteristics.xml to data folder
all karts from karts folder put to karts folder in your game folder

if you have stk installed via pkg manager on linux then you need root to change game files(your installtion of the game is probably in /etc/game/supertuxkart or something like that)
i would recommend to use stk from stk site which you can place whenever you want
