Make sure both your ball and plane have a collider attached and be sure that your ball is ABOVE the plane, otherwise it might just fall down. In addition to that follow Positive7’s comment. Let me know if it works or if you have other problems
EDIT
Also change ‘horizontal’ to ‘Horizontal’ and ‘vertical’ to ‘Vertical’, if you want to know the exact name of input axes just go to Edit->Project Settings->Input