You’ll need a popup, so create one from the WP admin at Breakdance > Popups. Set it to appear everywhere on your site, but don’t set any triggers for it.
Go to the page where you want the popup to open and open it up in Breakdance for editing. Find the button or link that will serve as your trigger.
Save your changes and exit to the front end of your site. Test the functionality by clicking the button or link. The popup will open in response.