You would usually use Make Refund/Debit (of method 'Opening balance debit'), and give it a suitable (possibly back dated) date, eg. first day of the previous month.
When considering the date, bear in mind that 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 first listed transaction on the statement would be a debit of description 'opening balance'. This could generate a query from your customer.
If you are not sure which is best, ask your book-keeper.
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). (you can also add a comment for your records). If you want the payment debit to appear on last month's statement, don't forget to back date the Tax Date so it's dated prior to the next statement month.
Naturally, if your customer was in Credit, you would choose Customer menu > Add payment/credit.