Hi,
Sorry for the issues you are having. I can help troubleshoot them with you.
I created an event in the Events Calendar and published on EventBrite.
I also created a coupon to make it free in Events Calendar.
I went back to the event on my site and added it as a promotional coupon, which changed the price to free and gave me a registration button instead of paypal.
I then clicked on the green Registration button and it opened a new page for me to register.
Is it not opening that page for you? I see in the event on my site there is still a loading graphic over the register button.
That button is controlled by the iframe from Eventbrite so there is no coding we can add to the plugin to change how coding works in the iframe.
Can you provide a link to the free event as a private reply so I can take a look and see if there is something that can be done.
Let me know and we can go from there.
Thanks