Hi,
You can also send email notifications by using workflows. First, make sure to promote the fields in your form as columns into your library so that you can use form values in your worklow.
You will need SharePoint Designer to create your workflow. There are a number helpful articles that have steps/guidelines for that - here's one: http://office.microsoft.com/en-001/sharepoint-designer-help/workflow-example-send-a-notification-message-HA010182908.aspx.
Based on your description, I imagine you would need to promote the Status (Approved/Rejected) and maybe the Manager fields. And then in your workflow, you will have a condition so that it only runs if Current Item > Status = Rejected; the action would send email to the Creator of the form (I think there's this option in there but please check).
Hope that helps!