Create a new Notification() and show it with window.showNotification().
Like so:
Notification n = new Notification("Remember!");
n.setDelayMsec(5000);
n.setPosition(Notification.POSITION_TOP_LEFT);
getMainWindow().showNotification(n);
This will show a notification for 5 seconds (after you move the mouse or press a button) in the upper left.
See Window.Notification for more info - you can configure some other stuff as well, such as stylename, icon etc.
A bit off-topic:
From a usability standpoint, you should avoid getting too crazy with icons and such, though - the notifications are designed to interfere as little as possible and draw just as much attention as needed, not more. For a great article on the subject, read Aza Raskin’s article: http://www.humanized.com/weblog/2006/09/11/monolog_boxes_and_transparent_messages/