Wordpress - Create an Info/Help message at top of a custom post type

The hook you are after is admin_notices. This is fired at the top of every admin page.

If you wish to restrict the notice to certain pages you can use: get_current_screen() to get the current screen.

 $screen = get_current_screen();

You can wrap the notices in div elements with class 'error' or 'updated' to get the red, or yellow styled notices.

For instance:

function wpse51463_admin_notice(){
      $screen = get_current_screen();

    //If not on the screen with ID 'edit-post' abort.
    if( $screen->id !='edit-post' )
        return;

      ?>

      <div class="updated">
        <p>
        A notice on the post list screen
        </p>
      </div>

      <div class="error">
        <p>
        An error message
        </p>
      </div>

     <?php

 }
 add_action('admin_notices','wpse51463_admin_notice');

For a plug-ins settings page (nested under the Settings tab) the screen id should be settings_page_{page-name}. You can of course determine the id of a page by using the above to print the current screen id of every page.

If you want something like this:

Then here's something that does the job:

add_action('admin_footer','print_mynote');
function print_mynote(){
    global $typenow,$pagenow;
    if  (in_array( $pagenow, array( 'post.php', 'post-new.php' ))  && "Post_Type_Name" == $typenow ) {
        ?>
        <SCRIPT TYPE="text/javascript">
            jQuery(document).ready(function(){
                var myDiv = jQuery('<div>');
                myDiv.css("border","1px dashed #000000");
                myDiv.css("padding","5px");
                myDiv.css("background","lightyellow");
                myDiv.css("width","70%");
                myDiv.html("PUT your info help message here....");
                jQuery(".wrap").find("h2").after(myDiv);
            });
        </SCRIPT>
    <?php
    }
}

Just make sure you change Post_Type_Name to the name of your post type, and change PUT your info help message here.... to your actual message.