How do add force from bullet on death?

This depends a lot on how your character is organized, how you're applying damage to it, how you're replacing it with the ragdoll etc. Based on the old-and-good-but-no-more-available FPS Tutorial, the easiest way to implement this is to apply forces to the object hit with AddExplosionForce: this distributes the forces among the ragdoll rigidbodies according to the distance, thus points nearest to the impact point will be more affected. The script below was adapted from the above mentioned FPS Tutorial (ExplosionAdvanced.js) and produced very reasonable results - attach it to the bullet:

 var impactRadius: float = 3.0;
 var impactPower: float = 50.0;
 var damage: float = 10.0;
 
 function OnCollisionEnter(coll: Collision){
     var hit: ContactPoint = coll.contacts[0]; // get contact info
     // apply damage to the object hit
     coll.transform.SendMessageUpwards("ApplyDamage", damage, SendMessageOptions.DontRequireReceiver);
     // if killed, the enemy now has been replaced by a ragdoll - get its colliders in an array:
     var colliders: Collider[] = Physics.OverlapSphere(hit.point, impactRadius);
     // check if there are rigidbodies among them...
     for (var col: Collider in colliders){
         var rigid: Rigidbody = col.rigidbody;
         if (rigid && !rigid.isKinematic){ // if it's a rigidbody...
             // apply a force inversely proportional to the distance from the hit point:
             rigid.AddExplosionForce(impactPower, hit.point, impactRadius);
         }
     }
     // destroy the bullet
     Destroy (gameObject);
 }

This script expects that the enemy has a function ApplyDamage, which will hurt it and, when killed, replace it with a ragdoll. If you're basing your scripts in the FPS Tutorial, the enemies probably have the CharacterDamage.js script. If not, that's the CharacterDamage script:

 var hitPoints = 100.0;
 var deadReplacement : Transform;
 var dieSound : AudioClip;
 
 function ApplyDamage (damage : float) {
     // We already have less than 0 hitpoints, maybe we got killed already?
     if (hitPoints <= 0.0)
         return;
 
     hitPoints -= damage;
     if (hitPoints <= 0.0)
     {
         Detonate();
     }
 }
 
 function Detonate () {
     // Destroy ourselves
     Destroy(gameObject);
     
     // Play a dying audio clip
     if (dieSound)
         AudioSource.PlayClipAtPoint(dieSound, transform.position);
 
     // Replace ourselves with the dead body
     if (deadReplacement) {
         var dead : Transform = Instantiate(deadReplacement, transform.position, transform.rotation);
         
         // Copy position & rotation from the old hierarchy into the dead replacement
         CopyTransformsRecurse(transform, dead);
     }
 }
 
 static function CopyTransformsRecurse (src : Transform,  dst : Transform) {
     dst.position = src.position;
     dst.rotation = src.rotation;
     
     for (var child : Transform in dst) {
         // Match the transform with the same name
         var curSrc = src.Find(child.name);
         if (curSrc)
             CopyTransformsRecurse(curSrc, child);
     }
 }