It is strongly recommended that you use the Google Chrome browser when using ClassDex. ClassDex utilizes modern browser functionality that just works best in Google Chrome.
Before using the program, you should visit the settings page under the ClassDex menu and make changes to customize the program to your needs.
If you want to be able to take online payment for your classes, you will also need to install the Cart66 plugin (Pro or Lite versions will both work), which is also available from the Plugin menu in your WordPress Dashboard. Cart66 needs some setup as well to integrate it with your payment gateway. Detailed instructions from the developers of Cart66.