How can I drop an object at will?

I am making a very simple "Pachinko" style game that involves a player moving a ball across the top of the screen until they find a space they want to drop the ball. The ball hits various bumpers/objects, and reaches the bottom where it should collide with pins. Knock over all the pins to win. Simple right? I am using C# to script the controls, and so far am able to move the ball back and forth, but cannot get the ball to drop. I have gravity on as well as isKinematic to prevent the ball from dropping upon starting the game. I want to turn isKinematic off when the player presses the "Down Arrow" key. However, I am getting a compile error. So far all attempts to correct this have failed. Can someone please help me with this? Here is the script I have crafted thus far:

using UnityEngine; using System.Collections;

public class dragNdrop : MonoBehaviour {

 public float ballSpeed = 1f; 
 public Rigidbody rb; 

 private Vector3 playerPos = new Vector3 (0, 4, 0); 

 // Use this for initialization 
 void Start () 
 { 
     rb = GetComponent<Rigidbody> (); 

 } 

 // Update is called once per frame 
 void Update () 
 { 
     float xPos = transform.position.x + (Input.GetAxis ("Horizontal") * ballSpeed); 
     playerPos = new Vector3 (Mathf.Clamp (xPos, -8f, 8f), 4f, 0f); 
     transform.position = playerPos; 
 } 

 void LateUpdate () 
 { 
     if (Input.GetButtonDown (KeyCode.DownArrow))
     { 
         rb.isKinematic = false; 
     }
     else
     {
         rb.isKinematic = true;
     }
 } 

}