Just follow these steps:
- Open SparkIV, browse to pc/models/cdimages and open up the vehicles.img
- Import the files , which should be renamed to 'dukes' (.wft and wtd) if you want to replace that one
- Then hit Save and exit sparkiv (don't use the rebuild option)
- You might want to change the handling like described in the readme, but the car itself should work ingame
You don't need a texteditor btw, just select 'open with' and use notepad to open it.