For the linux archive, extract it and run run_game.sh inside
For the windows archive, extract it and run run_game.bat inside
If you want to build STK 1.5, get the SRC package.
Note: If you get a "Not all assets found - aborting" error, please re-download, the problematic packages have been fixed. Alternatively, you can manually go in the game's "data" folder. Copy all the content of the "stk-assets" folder within directly in the "data" folder (for example /data/stk-assets/tracks should become /data/tracks). We apologize for the inconvenience.