Creating Accounting Derivation Rule in Receivables

Am trying to create a new Accounting derivation Rule to derive receipt accounting by substituting segment # 2 of the Accounting Flex field with the value contained in the Receipt Header Descriptive Flex field.  However, I am not able to select the second segment from the LOV, what is available are the: Balancing segment, Cost Center, Intercompany, Management and Natural account segments. What I really need to select is the #2 segment which is the division. Am I missing any step? What do I need to do to make all the accounting flex field segment available for selection from the LOV when creating