Are you putting the cars in the right folder?(slrr/cars/racers) if you have it might be because you have started a new game so you would have to buy one of the stock slrr cars and they should start to appear, if that didn't work there is always the car shop in the shop menu *top* but they will cost a bit..
the duhen and einwagen cant be removed if i'm correct (don't kill me if i'm not) but there is a script to make them not show up as much in the dealers i think so it's worth checking that out.
