I need one of my prefabs to be deleted after a few seconds

This is not a wise approach to scene or instance management.

First, objects like projectiles should generally be pooled for efficiency; plenty of info about this concept floating around.

Often the pooling concept gives you a convenient chance to store and manage both prefabs and instances. Using the Find() family of methods should always be a last resort; they're expensive.

If you don't want to get into all this right now, just create a script on your bullet prefab that self-destructs after N seconds:

Do not keep your prefab alive in the scene; it should only exist in your project's assets as a prefab.

 void Start(){
  Destroy(gameObject, DestroyTime);
 }

This is a nice answer. You should accept it.

I did what you said, and changed the code:

Bullet Destroy:

 #pragma strict
 
 var DestroyTime : float;
 
 function Start () {
     Destroy(gameObject, DestroyTime);
 }

Bullet Spawn:

 #pragma strict
 
 var bulletSpeed : float;
 
 function Update () {
     if (Input.GetMouseButtonUp(0)) {
         var bullet = Instantiate(gameObject,GameObject.Find("BulletSpawn").transform.position,Quaternion.identity);
         bullet.rigidbody.AddForce(transform.forward * 2000);
 
     }
 }

I moved both the bullet spawn and bullet destroy scripts to the prefab, after deleting the prefab from the scene. Now the Projectiles do not spawn at all. Once I moved the scripts, i changed the destroy time to 7 seconds and the speed to 2000, but it still doesn't spawn them. What do I do to fix this?

Also, if the above comment cannot be fixed for some reason, what is "pooling" like you mentioned earlier?

This entire question was asked because of this youtube series: http://adf.ly/1AO5Cm (episode 5 is where i encountered the problem)

You should spend some time looking over Unity's on-site tutorials and lessons. A bullet should not spawn itself, nor should it need to Find() the location from which to spawn. If this approach was suggested by a youtube video, I wouldn't recommend following their advice.

Instead, some other object (like a Player or a Gun) should handle spawning by keeping a reference to the bullet prefab.

 // on your gun/player/turret/whatever script, assign the prefab by click-n-dragging it from your assets folder
 public GameObject bulletPrefab;

Pooling is a concept used to reduce the cost of spawning objects at runtime. There's a lot written on the subject, so finding more information should be easy. Search for "Unity Object Pooling" or something. Don't worry about this yet though; learn your fundamentals first.