In the SCLogin parameters Advanced section, please try enabling the "Load JQuery" option. There is a bootstrap Javascript that is required to be loaded on your site before the modal popup will work. If it is not already included by another extension or your template, this behavior can happen.
Additionally, please check the error console in your browser's developer tools to make sure there are no Javascript errors on your page on load or that occur when the button is clicked. If the JQuery option doesn't work, this is usually the next usual common cause.
Please try both of these suggestions and let us know if you're still having issues afterwards.
Thanks,
Melissa