In the interest section unless you are also getting principal payments ... 

Enter it like you got a 1099 INT for interest and put the amount in box 1

Enter a 1099-Int under

Federal

Wages & Income

Interest and Dividends

Interest on 1099-INT - Click the Start or Update button

Interest and principle payments for installment sales go here :  

@VolvoGirl 

There is a separate place for seller held loans ...see my screenshot.