Open the challenge checker:
Click "Link ..." and copy either of the three HTML code snippets that are available (with previews on left). Then open the cache listing:
Click "Edit", scroll to the "Description" section, click the "Source" button, which will show you the source. Then paste the snippet from above to the listing.
Click "Save and view" and it is done.
Repeat pasting the snippet elsewhere in the page if you are not happy with the result.