You would usually add a Payment Debit (of method 'Opening balance'), and give it a suitable (possibly back dated) date, eg. first day of the previous month. When considering the date, bear in mind if your next statement is going to be for say, July. then if you date the opening balance transaction as 30 June the statement will simply show 'balance brought fwd' at the top. Whereas if dated 1st July then the balance brought forward would be 0.00 and the first listed transaction would be a debit of description 'opening balance'. If you are not sure which is best, ask your book-keeper.
Alternatively, raise an invoice. (New job > add rows, then Action Bookout Complete, choosing a tax date).
Adding an Opening Balance (debit)
|•||Select a customer onto the Main Form|
|•||From the Customer menu, choose Make Refund/Debit..|
Enter the refund/debit details, choosing an appropriate 'Payment Method'. (eg Opening Balance Debit). (If you also add a comment it will be easier to identify in some lists). If you want the payment debit to appear on last month's statement, don't forget to back date the Tax Date.
Naturally, if your customer was in Credit, you would choose Customer menu > Add payment/credit.