Display warning on next button before moving on

I have a multipage form, where I would like to display a warning (preferably in a lightbox), that pops up when the user clicks the next button. We need to give the user a notice when only one checkbox is selected from multiple checkbox inputs. The user should then be able to go back and check more off or simply continue to next page.

I have tried to do some jquery, to prevent the submission of the formpage when clicking Next, but I can't get to stop.
Here is what I have:

jQuery(document).bind('gform_post_render', function(event, form_id, current_page){
	jQuery('.gform_next_button').click(function(){
		if(jQuery('.myinputclass input:checked').length == 1) {
			confirm("Are you sure?");
		}
		return false;
	});
});

Might someone have an idea to how I could solve this?

Thanks
Vayu

Hi Chris.
Well,not really. The site is online, but you gotta sign up for it to see the form. You can do this and it requires not cost or credit card. However, this JavaScript code I have added above is not on the production site, it's only on my local machine development site.
The public site is here where the form is embedded. http://forsikringsdeal.dk/ny-eftersporgsel/ But like I said, you can only see the form if logged in.