First, make sure Interest Expense is enabled by navigating to: General Revenues > Accounts Receivable > Accounts Receivable Settings > Setup > Other Options

Click Update
Check the box that says Automatically calculate interest expense after
Enter a number of days in the first field that after which interest expense should accrue
Enter the required interest expense rate in the second field
Click Accept
Exit Bill Refund Processing (if it is already open) and re-open the program. The Interest Expense field should now be automatically calculating
Make sure to fully exit Accounts Receivable Settings by using the Back buttons in the Ribbon Menu

Browser not supported