Yes, both simple and advanced integration can use SSL to make sure personal information from your clients is secure.
In case of advanced integration, SSL detection is done automatically (if the plugin itself resides on SSL-protected website, all data exchange is done with the SSL protected-area of checkin24-7).
In case of simple integration, you need to make sure you link to https://www.checkin24-7.com instead of the default http://www.checkin24-7.com. By replacing the protocol (http:// with https://) you ensure your users will be redirected to the secure area of checkin24-7.