you must assign the texture in the uv-image editor:
assight some material to your kart
select your kart, go into editmode and click on the plus-symbol in the uv-image-editor(this adds a new image)
if you do this, you have a texture assigned, but this is black at the moment
unwrapp your kart.
If you bake now the materials: you get your texture, save it
here is a video, how to bake material/testures
http://www.youtube.com/watch?v=Tj-S5QAac3Uyou don't have to add a material with that texture in the material/texture-panel
you can also look here:
http://supertuxkart.sourceforge.net/New_kart_tutorial_2