Confluence no longer has the 'Add' button: https://confluence.atlassian.com/display/ONDEMANDKB/Add+Page+Button+is+Missing
As best as I can tell, that is where one found access to the diagramly plugin.
How do I get to it now?
Thanks, that works, though there was another point of confusion, which is that the page in which a diagram is inserted must be saved first before the diagram can be edited. Not sure if this is documented somewhere, but if not I think it would be a stumbling block for new users, who might naturally try to test the plugin, as I did, on a newly created test page first. That one must first save the page before editing the diagram is not intuitive.