Action errors

how to set errors in Action
Lets say I was doing like

/** Get a handle to the LoginActionForm */
LoginActionForm loginForm = (LoginActionForm) form;

/** Retrieve Username and Password */
String lUsername = loginForm.getTxtUsername();

/** Get ActionErrors instance */
ActionErrors lErrors = new ActionErrors();
if (!lUsername.equals("admin"))
lErrors.add("loginError",new ActionError("/root/variables/error/username_notfound"));

The element "/root/variables/error/username_notfound" contains the detailed error message in variables_en.xml

Then I add this error object to request.
The error object is serialized but only Element is created...where is the value??

<error>
    <loginError/>
</error>

What is the correct way to do it?

TIA
-Nitin