How to associate prefab with a variable in C#?

I'm following this tutorial to make a simple FPS except I do the code in C#. It says to instantiate a missile (rigid body sphere) I need to associate the Missile prefab with projectile variable and when the code is in JS it can be done by dragging and dropping since I see all the information about the script in the Inspector section. When it's C# Inspector only shows that my script is attached to the Launcher, no variables and all. How do I associate Missile prefab with projectile variable?

Just in case, here's my code:

using UnityEngine; using System.Collections;

public class MissileLauncher : MonoBehaviour {

Rigidbody projectile; int speed= 20; void Update (){ if(Input.GetButtonDown("Fire1")) { Rigidbody instantiatedProjectile=(Rigidbody)Instantiate (projectile,transform.position,transform.rotation); instantiatedProjectile.velocity=transform.TransformDirection(new Vector3(0,0,speed)); Physics.IgnoreCollision(instantiatedProjectile.collider,transform.root.collider); } } }