How to install add-ons
Note that this information is outdated. SuperTuxKart 0.7.2 and later provide their own in-game addon manager which manages downloads and installation of addons.
SuperTuxKart has a standardized format for karts and tracks, allowing anyone to create their own if they wish. Below are instructions on how to install addons on each platform.
On Windows, the tracks and karts are stored in the data subdirectory of the STK installation. (Typically C:\Program Files\SuperTuxKart\) Inside the data directory, you will find, among other things, a karts and tracks folder. These contain subfolders representing each kart and track, respectively.
To install either, simply copy the folder you got/downloaded into the appropriate folder.
Make sure you have sufficient rights to put the folders in the proper directories
Place the kart folders from the folder karts from the addon download in the directory usr/share/games/supertuxkart/data/karts
The directory to place the tracks is usr/share/games/supertuxkart/data/tracks
Mac OS X
Right-click on the SuperTuxKart application, and select Show Package Contents. Open the Contents directory, then Resources, then data (possibily twice depending on the version).
You are then in the data directory of SuperTuxKart. Simply add tracks to the /track directory, and karts to the /kart directory there.