Hi Chantal!
Thanks for reaching out to us and also for using our plugins! 🙂
That “Register for this event” button is not something we add to our plugin. It seems to be the theme you are using is what it’s adding it. That’s why when switching to a default theme like twentyfifteen you don’t see the button anymore.
I would recommend reaching out to the theme authors for support on this.
I am sorry we can’t be of help here, but there isn’t much we can offer for that registration feature because we do not make it.
Good luck with your project!
Victor