To unlock more rows of vehicles, you must obtain a number of X marks on the cups. You get an X mark for each difficulty you clear a cup on (If you clear a higher difficulty before clearing a lower one, you will receive an X for all difficulties up to the one you cleared).

Code - Effect

2nd row of vehicles - 3 X marks
3rd row of vehicles - 6 X marks
4th row of vehicles - 9 X marks
5th row of vehicles - 12 X marks