error : com.sun.tools.attach.AgentInitializationException: Agent JAR loaded but agent failed to initialize

We are trying to make a **simple** hello World agent class but the agent cannot be initialized

ex:
====
VirtualMachine vm = VirtualMachine.attach("4520"); //PID of the tested application is hardcoded
String agent = "c:\\temp\\MYAgent.jar";
vm.loadAgent(agent, null);

ex: (the AgentClass)
=====================
import java.lang.instrument.*;
public class MYAgent {

public static void premain(String args, Instrumentation inst) throws Exception {
JOptionPane.showMessageDialog(null, "Hello from premain agent!");
}

public static void agentmain(String args, Instrumentation inst) throws Exception {

JOptionPane.showMessageDialog(null, "Hello from premain agent!");
}
}

The manifest look like this :
Manifest-Version: 1.0
Premain-Class: Com.MyAgent
Agent-Class: Com.MyAgent
Created-By: 1.6.0_06 (Sun Microsystems Inc.)

Note : I tried with premain, agentmain etc... any possible way and it's not working

We always get the error : com.sun.tools.attach.AgentInitializationException: Agent JAR loaded but agent failed to initialize

How the hell the loaded agent jar can be correctly initialized ??? I know there is many possible cause but we tried almost anything

Thanks

Michel