Registering through the site just helps us with an idea of numbers but will not replace the paper (insurance exercise) on the day so do not worry if it does not work.
When you have logged in go to the event page and at the bottom of the page you should see this.
Tick the box and click register button.
However it would be really helpful if you can give details when you post issues.
Please include, operating system, browser and error message if you can.
This will help try and sort out the issues for others. Sorry for the inconvenience.