Can't get Stepper to Work in DrScheme

[Jeffrey Stephens <jsteve17@tampabay.rr.com>]

I am using DrScheme version 103.5d104.  I am trying to use the Stepper to
step through the following code which I have entered into the Definitions 
window and then clicked on Execute:       --code is from HtDP Finger 
Exercises 3.3.6

;;fahr->cel: number -> number
;;convert temperature in fahrenheit to celsius
(define (fahr->cel temp)
  (*
   (- temp 32)
   (/ 5 9)))

;;cel->fahr: number -> number
;;covert temperature in celsius to fahrenheit
(define (cel->fahr temp)
  (+ (* (/ 9 5) temp) 32))

;;I: number -> number
;;convert temperature in fahrenheit to celsius and back
(define (I f)
  (cel->fahr (fahr->cel f)))

When I click on Stepper the Stepper window appears and has the code in the 
upper part followed by two horizontal blue lines with a red arrow between 
these two blue lines.  I can't seem to figure out how to move the red arrow
into the upper part of the window so I can step through the code.  I searched 
through the documentation but couldn't find anything.  Should I be including 
something in the definitions window other than the code?