In the add loan screen set the Interest Method to fixed Amortization and check the box for the Custom Payment Schedule. Navigate to the custom payment tab and enter the payment schedule for the loan. Navigate back to the main tab and you will see the check box to "Use Implicit Interest Rate" in the bottom left of the window. Place a check mark in the box and NLS will calculate what the interest rate needs to be based off of the loan parameters and the payment schedule.
