2D explosion that targets specific gameobjects

I don't know how to make it via Playmaker, but in C# it's pretty simple. Create an overlap sphere at the point of explosion, and gather all the colliders of tag "anytagthatyouwant" in an array, And then simply loop that array and do whatever you want with them (like adding an ExplosionForce).

A link to Explosion Force, http://docs.unity3d.com/ScriptReference/Rigidbody.AddExplosionForce.html

And a small change to the script for you, Here you go:

 void Start() {
         Vector3 explosionPos = transform.position;
         Collider[] colliders = Physics.OverlapSphere(explosionPos, radius);
         foreach (Collider hit in colliders) {
             if (hit.gameObject.tag =="yourtag" && hit.rigidbody)
                 hit.rigidbody.AddExplosionForce(power, explosionPos, radius, 3.0F);
             
         }