Just copy the plugin files into a new folder in the exporters-html5-plugins folder.
However, it stopped working because the domain got expired.
If you don't need thousands of Emails a month and can do with just a hundred/month, you could easily set it up through Zapier and their Webhooks Zap.
You send Zapier a POST request through the AJAX plugin which Zapier then turn into an Email. it's easy to set up, you can design the Mail much more comfortably than in the Construct 2 editor and it works flawlessly, but it's limited in scope. if you want more Emails, you would need to access the API of other mailing services. it would be a bit trickier though.