I just found this plugin which allows you to create custom options http://wordpress.org/extend/plugins/custom-options-plus/faq/
You can then place the php code to display the value of the option inside your dashboard for members who are logged in but not administrators. You can display the value as text wrapped in a div tag and style as you need.
Using the plugin I have linked to you can create two options one called ‘members’ another called ‘guests’, now that you have control of these variable values through your dashboard as the administrator the code below should display these messages accordingly to your members if they are logged in or not logged in.
An example php function may look like this
<?php
if ( is_user_logged_in() ) {
echo get_custom('members');
} else {
echo get_custom('guests');
}; ?>