This plugin is loading its CSS using the WP_PLUGIN_URL constant instead of the plugin_dir_url() function, so it triggers insecure content warnings on SSL pages. I've added a "fix" for this plugin to my SSL Insecure Content Fixer plugin, but you should make the following changes to avoid these warnings without the aid of my dirty kludge :)
* use the wp_print_styles action to enqueue your CSS, instead of printing it from wp_head action
* use plugin_dir_url() to get your plugin dir, instead of WP_PLUGIN_URL, so that it handles the transition to SSL (HTTPS)
Once you've released a version that fixes these, please let me know and I'll remove the "fix" for your plugin.