Notifications appear when the associated event occurs. Have an action due at 11:13 AM, that's when the Growl alert pops up.
You should be able to create an action, set its due date to one minute from now, and then wait a minute, and see the Growl notification appear.
You can see which events OmniFocus show notifications for by looking at the Growl preference pane: go to Applications -> OmniFocus -> Configure... -> Notifications.
You can also configure different Growl behaviors for each of these events.
-Dennis