Added an entry type for pending external items. The entry type is not available for selection and u

We are configuring AR to process items from an external billing system.  We will use the AR Distribution Code, entry type and reason to generate the accounting entries.  We added an entry type for pending external items.  The entry type is not available for selection and use by the pending external item.  What are the steps to add an entry type and reason for use with this process?