Correct, this option is not available under Inquisit 3 (you are looking at the Inquisit 5 documentation). But you can display the id nonetheless. Set up an additional <trial> element in your IAT and run that trial at the very end. Have that trial display the subject id via a standard <text> element and instruct your participants to note their ID down (copy & paste will NOT be possible).
/ stimulusframes = [1=idtext]
/ validresponse = (57)
/ items = ("Please write down your personal ID: <%script.subjectid%>")