I've got problems as well:

1. in the skin changing program, I save my skins, and it says:
Errors: carsdb; shellsbl; tpc;

2. I messed with the volume a bit, now, the volume is stuck at barely audiable, like nearly Zero.

Now, for modding questions:

1. Can you add your own skin into the skin editing program
2. How do you modify sounds
3. How do you edit level textures?