Where can I enter the balance carried forward?

Under the Accounting heading, the balance carried forward can be entered in the Account Statement tab for each balance sheet account, but this is only possible if the financial year is still open.

Select the desired balance sheet account. As soon as the "Pen" symbol is selected, a window appears in which the desired amount can be entered.  

It is also possible to enter the balance carried forward in the financial year tab.

In the following year, the final balance of the previous year is automatically transferred.