Use Check Return and find the Form or Schedule you want then the line number you want and CLICK on that line (or box). That should take you to the input you need. 

I am guessing you meant part II. You will need to check one of two boxes and then select from a drop down 

I added the "button" that does not seem to be there

That then gives you the choice of which line from part 3 the carryforward would have come from