Please check the suggestions made by Abhi and hope you are testing all these in Dev client only.
Apart from the above suggestions, the F4 dropdown in customer master for the Credit control area to appear, you need to made the following assignment.
Financial Accounting (New)-->Accounts Receivable and Accounts Payable-->Credit Management-->Assign Permitted Credit Control Areas to Company Code
and assign the permitted CCA to the company code in which the customer master has been extended.
Thanks,
Umesh