How you get the param sorta depends on where you are, 
You do not have to catch a request $param to achieve what you want to do here. You are just using the FlashMessenger helper to add a message to the stack. You then retrieve the message within the action where you want to show the message, then assign it to the view as I do in the successAction. Keep in mind that you can pass any $var or array of data by assigning it in the controller like: $this->view->var = $var; Within the view that will then be accessed as $this->var.
Since you asked about login I will show you how I usually do it. Not that its the best way.
My LoginController's index view holds the form:
    public function indexAction() {
    $form = new Zfcms_Form_Login;
    $this->view->form = $form;
     /*check for valid input
       authenticate using adapter
       persist user record to session
       redirect to original request URL if present*/
    if ($this->getRequest()->isPost()) {
        if ($form->isValid($this->getRequest()->getPost())) {
            $values = $form->getValues();
            $authAdapter = $this->getAuthAdapter();
            # get the username and password from the form
            $username = $values['username'];
            $password = $values['password'];
            # pass to the adapter the submitted username and password
            $authAdapter->setIdentity($username)
                    ->setCredential($password);
            $auth = Zend_Auth::getInstance();
            $result = $auth->authenticate($authAdapter);
            if ($result->isValid()) {
                # all info about this user from the login table
                # ommit only the password, we don't need that
                $userInfo = $authAdapter->getResultRowObject(null, 'password');
                # the default storage is a session with namespace Zend_Auth
                $authStorage = $auth->getStorage();
                $authStorage->write($userInfo);
                $session = new Zend_Session_Namespace('zfcms.auth');
                if (isset($session->requestURL)) {
                    $url = $session->requestURL;
                    unset($session->requestURL);
                    $this->_redirect($url);
                } else {
                    $this->_helper->getHelper('FlashMessenger')
                            ->addMessage('You were successfully logged in as ' . $userInfo->username);
                    $this->_redirect('/login/success');
                }
            } else {
                $this->view->message = 'You could not be logged in. Please try again.';
            }
        }
    }
}
In the success action we do this:
public function successAction() {
    if ($this->_helper->getHelper('FlashMessenger')->getMessages()) {
        $this->view->messages = $this->_helper
                        ->getHelper('FlashMessenger')
                        ->getMessages();
    } else {
        $this->_redirect('/login/success');
    }
}
In the view script we can do something like what I have below. The reason I do it this way is that sometimes I will pass only a single message in a controller, in this case I simply use: 
$this->view->message = 'message goes here';
Then catch them both if they are set in the view:
<?php 
    if(isset($this->message) || isset($this->messages)):
?>
<?php
if(is_array($this->messages))
{
    echo implode($this->messages);
} else {
    echo $this->message;
}?>
<?php 
endif 
?>