The short answer is:
To enable the Show Problem Source button, place source.pl as one of the files listed in loadMacros(.....) at the top of the source code for the question.
However this will only work on courses that have been enabled in the following ways:
In the webwork2/conf/webwork.apache-config file insert this snippet:
Insert the file https://math.webwork.rochester.edu/webwork2_course_files/webwork_presentations/show-source.html in the myCourse/html directories of the courses you wish to enable.